"1. Every time I attack a certain character, it runs away (which is probably wise if I have the advantage) and then logs out as soon as the melee timer permits."

Certainly permitted by the rules (which is why the log out timer exists), if it is a consistent pattern, it is relatively cheesy to do so, if the Imms aren't already aware of it, I'd report it so they can determine if it is going on a little to frequently or the character in question is dropping link rather than waiting out the timer.

"Even if I can strongly infer from the RP that the original plan was to run experience for two (real life)hours, if someone attacks the group and there's a merry chase and we win, they'll hang around for 10 minutes and then log out. Sometimes they even come back in 20 minutes to 2 hours, but they always log out. What's going on here?"

Some older players religiously log out after they are killed in such fashion to prevent themselves from deleting. That may be what is going on. Also, if they intended to rank for 2 hours, knowing that their previous ranking spot is no good, if they can't come up with alternatives, then their plans are ruined.