The description for Resurrection includes "You can resurrect someone killed by a death effect or someone who has been turned into an undead creature and then destroyed." (emphasis mine) That indicates to me that a) you cannot resurrect someone who has been turned undead until that undead creature has been destroyed and b) that it is possible to resurrect them once that has happened. This phrase is repeated verbatim on True Resurrection, so I'd say that no, this is not possible on a strict reading of the rules. Well, not without time travel, at any rate...
Since D&D's ghouls, designed by Gary Gygax, were strongly based on the critters of the same (Arabian) name invented by H.P. Lovecraft...
The D&D ghoul was inspired by the Lovecraftian critter of that name
and my own imagination. they first appeared in play in c. 1970 in the
Chainmail Fantasy Supplement table top games.
– Gary Gygax, August 12, 2005, EN World Q&A IX
...it seems a rather logical step to read HPL's relevant short stories - especially Pickman's Model and The Dream Quest of Unknown Kadath to get a view of how they behave.
Of course, the D&D ghoul has deviated from HPL's one over time, so you might want to check out further D&D sources as well: Dragon Magazine #252 (October 1998) had an article on their ecology, and there are quite a number of other publications dealing with them as well.
And, finally: it's your game. Read up on ghouls, and design and play them as you see fit, make them truly yours adapting them to your campaign world. Remember, you're not bound by anyone else's creativity. :)
As for your questions, my subjective answers would be:
Are they truly capable of elaborate communication (like a conversation), and willing to engage in some?
Yes, they are capable of elaboration, and are willing to engage,
especially if a PC is important for them for some reason
(ex-relative, or has something - information or goods - for them.)
Do they have clear memories of their former lives?
Yes, they do. Well, at least some of them do. HPL's ghouls
remember... as far as I can remember.
Do they feel genuine emotion, and possibly regret as to their lost life?
It depends on the individual, I guess. An inspired poet turned into a
ghoul will feel much more than, say, a bloodthirsty gladiator.
And any other detail relevant to their personalities.
See above. Read those two stories. They're good (I think. :))
Best Answer
No, the resurrected doesn't remember.
Quote from Speak with the Dead PHB 281
The soul of the dead body is off wherever those souls go, this spell is using the residual memories of the actual body to answer questions, so the resurrected have no idea if they've been interrogated or not