a) In the dispensation of a Buddha Sasna ( like the age we are presently living today , wherein the teaching is clear for one to strive forward) , can there be
human beings born in this age who are Bodhisattas , but completely bypass the teachings of the Buddha , and may or may not live a wholesome life .
A Bodhisatta is not guaranteed to become a Buddha until such a guarantee (niyata vivarana) is received from a living Buddha. So I'd guess a Bodhisatta who has not received such a guarantee may fall off the path at any given point - either through bad conduct or lack of zeal. Would a Bodhisatta avoid an existing sāsana while being in the human realm? Hmm.. I'd say possible, but unlikely, assuming the Bodhisatta is making progress in her career.
For either kind of Bodhisatta, since the Bodhisatta is a worldling, she is liable to breaking precepts; I think the jātaka stories have the Bodhisatta breaking all the five precepts except the fourth(lying). Being a worldling the Bodhisatta cannot avoid the bad realms: sometimes the Bodhisatta is an animal in the jātakas - so no chance of learning from any existing Buddha-sāsana in such situations.
b) Just as there can only be on Buddha at any given time , can there only be one Bodhisatta at one given time .
I don't think that would be the case. Even if we limit our consideration to Bodhisattas who have received niyata vivarana, a few such Bodhisattas existing simultaneously would be possible. e.g. Maitreya Bodhisatta and Gautama Bodhisatta would have existed simultaneously before the awakening of Gautama Bodhisatta.
That being said:
Pretty much all this is mostly guesswork based on writings about the Bodhisatta path, since a Bodhisatta path is not mentioned by the Buddha anywhere in the Suttas. I think the Āgamas may have one or two mentions, but the rarity of such a mention and that it doesn't exist in the other canons doesn't make it a very trustworthy source. Perhaps a clear-cut path doesn't exist for becoming a Buddha. Scholars like Dhammapāla(5th century?) used the various hagiographic information written about the Buddha and his disciples to create a framework for a Bodhisatta path. All of this is rather late work, inclduing the jātaka tales(some of which seem to be common Indian folklore). That's why it's hard to give any straightforward answers to your questions.
P.S. Searching for the Āgama material mentioning the Bodhisatta I found this sūtra: Ekottara Agama 20.6—Maitreya’s effort
And this message on a Pāli mailing list:
In SN(Sutta-nipata), Parayana-vagga, 16 pupils asked questions to Buddha. Ajita is the first one and Tisa Metteyya is he second. According to Chinese translation, The name of Matteya Bodhisatta is Ajita.
But in SN, it is described as two persons. First of all, reading SN you may find out what described about so-called Matteya Bodhisatta is different with what in SN. There is no reasons to assume they are the same people.
According to Agamas, there is no descriptions of [Matteya Bodhisatta] in Samyutta Agama, there is only
one sutta each in both Majjima Agama and Digha Agama talking briefly about [Matteya Bodhisatta], but having nothing to do with [Matteya Bodhisatta]'s special charateristics:[not praticing jhana, not totally-abandoning kilesa]. In both Majjima Agama and Digha Agama, this short description about [Matteya Bodhisatta] is related to descrptions of another Indian Myth--Cakkavatti-Sihanada(King of wheels). It should be treated as fairy story instead of history. There are seven to eight (longer) descriptions in Anguttara Agama. Due to strong flavor of Mahayana in Anguttara Agama, I do not want to quote them as bibliographical reference.