Having Dharma skills and knowledge does not necessarily translate to good attitude or actions... Its just like having a plate of salad in front of you, but if you're not going to use your hands and salad fork to eat, then what's the point?
Like what Aurore said about unrecognised Tulkus, I must agree on that point. There are many people out there that have good knowledge in Dharma due to their past studies in previous lives. Its not so much as they can remember everything, but it is easier to catch up compared to others (of course, there are exceptions). Even for Tulkus, there are 'classes' where some are higher and some lower... lower in the sense of attainments therefore if they were to be born into a condition that is very samsaric, it may pull them into a samsaric lifestyle despite having much untapped Dharma knowledge in them. This is very sad...
How fast we accumulate knowledge depends on our effort... how much wisdom we get out of that knowledge depends on our practice. I believe it is not the knowledge that we literally bring to our future lives, but it is the wisdom that we accumulated.
As for how much knowledge one needs to be a Buddha? That I dont know for sure... but if we look at it in a logical way, then all the knowledge and wisdom of the Lamrim, once internalized thus we will be on our way to Buddhahood. Once reached enlightenment, we are all the same... in the same mindstream as Shakyamuni himself.. thus Manjushri, Shakyamuni, Milarepa and all other enlightened beings are of the same mindstream and possessed equal qualities of compassion, only depicted in very different ways.
As for being in the formless realm... I don't think one can attain enlightenment in this realm... as far as I know, there's no Boddhisattvas that has ever been born in this realm.