Please allow me to begin by saying that there are no actual dogmatic statements from Church councils regarding this topic. I’m going off of the general approach of the Fathers of the Church, though even there not all patristic sources line up (see any discussion of Toll Houses as an example). There are also far better and more experienced priests and monks who know far more than I, but I will try my best to answer all your questions.
Regarding fallen angels, if these angels are in the very presence of God why would they fall from such perfection witnessing what is perfect?
That is the insanity of the demonic. They are at one time very crafty and very deluded. I don’t think anyone can safely say what it is psychologically like to be an angel, so your guess is as good as any. All we know is that some of them, approximately a third, fell with Satan.
How exactly does pride manifest itself in front of a presence of that which is perfect?
We may ask ourselves the same question when we read the Gospels and then fail to live according to their commands. We are not perfect, and neither are the angels. Their ‘perfection’ comes through obedience, while ours comes through union with our Lord Jesus Christ.
It kind of brings up the next question of "free will" in Heaven; is that even possible?
When you think about it, the concept of ‘free will’ is at one time preposterous, yet critical. To have a free will means that one is an independent entity. If we had no free will, we would be God’s machines or extensions of Him rather than our own beings.
It is preposterous to think that one would choose against the God who is all-knowing. However, we have that ability in order to be persons separate from God’s consciousness.
Would it (free will) cease to exist because knowing what suffering, sin etc is like we wouldn't want anything else but to be in commune with God.
On a reasonable level, yes, this is true. Functionally speaking, this is what we are here to do: to choose communion with God over selfish desires. This life is about coming to know the full implications of the free will, so that we will, in the end, choose freely to accept God’s will and thus retain our personhood while enjoying His love without compulsion.
Is it really possible for a practicing Christian to become possessed by a demon or demons?
‘Practicing’ is a tricky word, since lots of people ‘practice Christianity’ the wrong way. See my remarks about living out the Gospel.
For the sake of clarity, allow me to review the basics:
- demonic possession is rare, even amongst the ‘willing.’
- demonic possession requires an agreement with the possessed or, in the case of some children, the agreement of the parents/guardians (see the story of the slave girl in Acts for an example of this arrangement).
- demonic possession would make the regular participation in the Sacraments nearly impossible without some serious ‘secret sins’ (i.e. the person receives Sacraments, then engages in significant sins thus receiving in an unworthy manner) or a case of ‘perfect possession.’
- demonic possession is known because the possessed fights against it once the agreement is struck and the demon begins to work through the possessed. ‘Perfect possession’ occurs when the possessed does not struggle and willingly yields himself to the demons. This type of possession cannot be exorcized, since they are unwilling to have the demon(s) leave.
OK, I hope this answers everyone’s questions (for now).