[RPG] Is a creature aware of the circumstances surrounding its resurrection

He cannot cast the spell if the conditions for its casting aren't met.

This is distinct from him casting the spell and the spell failing, mind you. In order to cast the spell, he must be within the range specified by the spell, which in this case is touch. Now there is an exception to this in this spell, which applies if the body no longer exists. Given that the body is still in existence, I would say that the spell fails should he decide to try and cast it regardless but that the spell slot is not consumed. The reasoning behind this is that the range of the spell is part of its requirements for its casting, as much as any component, so casting True Resurrection without being in range would have the same result as casting it without a diamond: the process of casting it remains incomplete.

That said, if you are using the optional rule on Xanathar's Guide to Everything (p. 86), under "Invalid Spell Targets", the spell slot would be expended:

If you cast a spell on someone or something that can't be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended.

That said, any high-level cleric would be aware of this, and would most likely not cast the spell.

Yes, as long as that hair or blood was taken from the body after it died

The resurrection spell requires

So long as some small portion of the creature’s body still exists, it can be resurrected, but the portion receiving the spell must have been part of the creature’s body at the time of death.

Thus, as long as you have any part of the dead body, you can cast Resurrection on that part to bring them back. Even if the body was reduced to a fine paste by a Three Speed Robe of Blending on Liquefy setting, you can resurrect them from a sample of that paste, no matter how small.

And adventurer cannot, however, give a friend a piece of their hair while they are still alive and instruct that friend to resurrect them from it if they die, because that hair would not have been part of their body at the time of death.