The beings depicted are believed to be real, but the skulls and other scary stuff are symbolic. For example (from Images of Enlightenment), "his crown of five skulls represents the five poisonous delusions--anger, desire, ignorance, jealousy and pride--transformed into... wisdoms..." (emphasis added) and "the curved knife in his first right hand cuts ego-attachment..." "He tramples on an elephant-headed deity to symbolize his destruction and dispersal of great obstacles."
Even distinctly Vajrayana ideas and practices, such as imagining the world as a pure realm, are traceable to Mahayana Sutras, such as the Vimalakirti-nirdesa Sutra. Robert Thurman has commented on this in his translation of the sutra. Tibetan Buddhism really absorbed very little from Bon. It was more the other way around. As Vajrayana Buddhism, which originated in India at least as early as the 7th century, became successful in Tibet, Bon tried to compete by absorbing Buddhist elements.
The idea of more than one buddha is found in all Buddhist traditions. Theravada teaches that there have been other buddhas before our Teacher and that there will be others in the future. The Pali scriptures name at least 28. Seven are referred to often in the scriptures.
I think that the question of whether a buddha remains accessible or present after death is trivial, because it is not relevant to the question of overcoming suffering or dukkha, which is the whole point of the Eightfold Path. (Yes, Theravada teaches that buddhas are no longer present after death, although you wouldn't know this if you asked some of the laity in Thailand. Mahayana scriptures teach otherwise.)
As for the problem of fortune-telling, Theravada countries are not immune from this, despite having scriptures which reject it.
Sīlaṃ balaṃ appaṭimaṃ.
Virtue is a matchless power.
Virtue is the greatest weapon.
Virtue is the best adornment.
Virtue is a wonderful armor.
etaṃ buddhāna sāsanaṃ.
Refraining from all wrong-doing,
Undertaking the good,
Purifying the mind,
This is the teaching of the buddhas.
Dhammapada v. 183/14.5