Mahayana and Theraveda are of the same One Buddha Vehicle. Buddha uses One vehicle to convert the Bodhisattvas, according to His own words in the Lotus Sutra. The Buddha is not guilty of lies or falsehood when it comes to His preaching, therefore everything you say is correct, and it is okay
just to stick to the Theravada style or Buddhism, just understand that it is a foundation of Mahayana, without Theravada there would be no Mahayana and in it's origin is Buddhahood and it's good roots. See here:
-The Lotus Sutra, Chapter Two: Expedient Means
In the Buddha lands of the ten directions
there is only the Law of the one vehicle,
there are not two, there are not three,
except when the Buddha preaches so as an expedient means,
merely employing provisional names and terms
in order to conduct and guide living beings
and preach to them the Buddha wisdom.
The Buddhas appear in the world
solely for this one reason, which is true;
the other two are not the truth.
Never do they use a lesser vehicle
to save living beings and ferry them across.
The Buddha himself dwells in this Great Vehicle,
and adorned with the power of meditation and wisdom
that go with the Law he has attained,
he uses it to save living beings.
He himself testifies to the unsurpassed way,
the Great Vehicle, the Law in which all things are equal.
If I used a lesser vehicle
to convert even one person,
I would be guilty of stinginess and greed,
but such a thing would be impossible.
If a person will believe and take refuge in the Buddha,
the Thus Come One will never deceive him,
nor will he ever show greed or jealousy,
for he has rooted out evil from among the phenomena.
Therefore throughout the ten directions
the Buddha alone is without fear.
I adorn my body with the special characteristics
and shine my light upon the world.
I am honored by numberless multitudes
and for them I preach the emblem of the reality of things.
Shariputra, you should know
that at the start I took a vow,
hoping to make all persons
equal to me, without any distinction between us,
and what I long ago hoped for
has now been fulfilled.
I have converted all living beings
and caused them all to enter the Buddha way.
As regards to how Buddha acheived Enlightenment in a distant past life according to His own words is written here in the Sixteenth Chapter of the Sutra. It is a very important read:
-The Lotus Sutra, Chapter Sixteen: The Life Span of the Tathagata
The Lotus Sutra
At that time the World-Honored One, seeing that the bodhisattvas repeated their request three times and more, spoke to them, saying: "You must listen carefully and hear of the Thus Come One's secret and his transcendental powers. In all the worlds the heavenly and human beings and asuras all believe that the present Shakyamuni Buddha, after leaving the palace of the Shakyas, seated himself in the place of practice not far from the city of Gaya and there attained annuttara-samyak-sambodhi. But good men, it has been immeasurable, boundless hundreds, thousands, ten thousands, millions of nayutas of kalpas since I in fact attained Buddhahood.
"Suppose a person were to take five hundred, a thousand, ten thousand, a million nayuta asamkhya thousand-million-fold worlds and grind them to dust. Then, moving eastward, each time he passes five hundred, a thousand, ten thousand, a million nayuta asamkhya worlds he drops a particle of dust. He continues eastward in this way until he has finished dropping all the particles. Good men, what is your opinion? Can the total number of all these worlds be imagined or calculated?"
The bodhisattva Maitreya and the others said to the Buddha: "World-Honored One, these worlds are immeasurable, boundless--one cannot calculate their number, nor does the mind have the power to encompass them. Even all the voice-hearers and pratyekabuddhas with their wisdom free of outflows could not imagine or understand how many there are. Although we abide in the stage of avivartika, we cannot comprehend such a matter. World-Honored One, these worlds are immeasurable and boundless."
At that time the Buddha said to the multitude of great bodhisattvas: "Good men, now I will state this to you clearly. Suppose all these worlds, whether they received a particle of dust or not, are once more reduced to dust. Let one particle represent one kalpa. The time that has passed since I attained Buddhahood surpasses this by a hundred, a thousand, ten thousand, a million nayuta asamkhya kalpas.
"Ever since then I have been constantly in this saha world, preaching the Law, teaching and converting, and elsewhere I have led and benefited living beings in hundreds, thousands, ten thousands, millions of nayutas and asamkhyas of lands.
"Good men, during that time I have spoken about the Buddha Burning Torch and others, and described how they entered nirvana. All this I employed as an expedient means to make distinctions.
"Good men, if there are living beings who come to me, I employ my Buddha eye to observe their faith and to see if their other faculties are keen or dull, and then depending upon how receptive they are to salvation, I appear in different places and preach to them under different names, and describe the length of time during which my teachings will be effective. Sometimes when I make my appearance I say that I am about to enter nirvana, and also employ different expedient means to preach the subtle and wonderful Law, thus causing living beings to awaken joyful minds.
"Good men, the Thus Come One observes how among living beings there are those who delight in a little Law, meager in virtue and heavy with defilement. For such persons I describe how in my youth I left my household and attained anuttara-samyak-sambodhi. But in truth the time since I attained Buddhahood is extremely long, as I have told you. It is simply that I use this expedient means to teach and convert living beings and cause them to enter the Buddha way. That is why I speak in this manner.
"Good men, the scriptures expounded by the Thus Come One are all for the purpose of saving and emancipating living beings. Sometimes I speak of myself, sometimes of others: sometimes I present myself, sometimes others; sometimes I show my own actions, sometimes those of others. All that I preach is true and not false.
Why do I do this? The Thus Come One perceives the true aspect of the threefold world exactly as it is. There is no ebb or flow of birth and death, and there is no existing in this world and later entering extinction. It is neither substantial nor empty, neither consistent nor diverse. Nor is it what those who dwell in the threefold world perceive it to be. All such things the Thus Come One sees clearly and without error.
"Because living beings have different natures, different desires, different actions, and different ways of thinking and making distinctions, and because I want to enable them to put down good roots, I employ a variety of causes and conditions, similes, parables, and phrases and preach different doctrines. This, the Buddha's work, I have never for a moment neglected.
"Thus, since I attained Buddhahood, an extremely long period of time has passed. My life span is an immeasurable number of asamkhya kalpas, and during that time I have constantly abided here without ever entering extinction. Good men, originally I practiced the bodhisattva way, and the life span that I acquired then has yet to come to an end but will last twice the number of years that have already passed. Now, however, although in fact I do not actually enter extinction, I announce that I am going to adopt the course of extinction. This is an expedient means which the Thus Come One uses to teach and convert living beings.
"Why do I do this? Because if the Buddha remains in the world for a long time, those persons with shallow virtue will fail to plant good roots but, living in poverty and lowliness, will become attached to the five desires and be caught in the net of deluded thoughts and imaginings. If they see that the Thus Come One is constantly in the world and never enters extinction, they will grow arrogant and selfish, or become discouraged and neglectful. They will fail to realize how difficult it is to encounter the Buddha and will not approach him with a respectful and reverent mind.
"Therefore as an expedient means the Thus Come One says: 'Monks, you should know that it is a rare thing to live at a time when one of the Buddhas appears in the world.' Why does he do this? Because persons of shallow virtue may pass immeasurable hundreds, thousands, ten thousands, millions of kalpas with some of them chancing to see a Buddha and others never seeing one at all. For this reason I say to them: 'Monks, the Thus Come One is hard to get to see.' When living beings hear these words, they are certain to realize how difficult it is to encounter the Buddha. In their minds they will harbor a longing and will thirst to gaze upon the Buddha, and then they will work to plant good roots. Therefore the Thus Come One, though in truth he does not enter extinction, speaks of passing into extinction.
"Good men, the Buddhas and Thus Come Ones all preach a Law such as this. They act in order to save all living beings, so what they do is true and not false.
is considered by many to be a Mahayana Sutra, but it contains great wisdom that the Buddha Himself preached, with useful and powerful means for one to attain Supreme Perfect Enlightenment, and in it's Teachings one can reach Buddhahood, it is the Buddha's prescribed method as He says within it's pages. Whether you consider yourself a Theravada or Mahayana Buddhist, or both, it is a great practice to read it. I once encountered a Theravada Buddhist monk who used the practice of meditation and Nichiren Buddhist chanting of Nam Myoho Renge Kyo
, which translates to "I Devote Myself To The Mystic Law Of The Lotus Sutra", to achieve Enlightenment, and the day He achieved Enlightenment He was the happiest person on Earth. I asked Him a question and in reply to the contents of my question He told me that everybody Loves, all beings Love, even the most fallen ones, even if just a small amount. There is that old John Lennon quote "Love is needing Love"
. So truly, the goal of life is Metta, and the unequivocal path to Buddhahood.