This entirely depends on your game, and your gaming group.
What you are referring to (acting as yourself instead of your character) is called "meta-gaming" and involves making decisions that are outside of the purview of your character's personality or knowledge. Whether or not this is an acceptable practice depends in part on your group and in part on the level or realism/seriousness of your game.
If your group is a lighthearted, we are playing this to have fun type group and you are playing in a system that focuses more on mechanics and tactics than on actual role-playing, then meta-gaming may be ok. You are likely free to make, at least in some limited sense, decisions that would not be likely/possible for your character to make, either because you personally feel a certain way, or because you have knowledge your character does not.
However, if you are playing in a more serious group, or more realistic setting where role-playing is the primary influence then it would be inadvisable to make decisions your character would not make, or act on knowledge your character does not have.
These are not discrete decisions either. Most groups fall somewhere in the spectrum between heavy story influence and heavy tactical influence. Systems do as well, although I feel this is more highly dependent on group dynamics and expectations than it is on the system you are playing in.
Sometimes role-playing your character leads to suboptimal (even abysmal) outcomes, this goes somewhat against our typical gamer mentality (I know it does mine), but is part of what makes RPGs special. You can have a ton of fun, even when your character dies horrible.
Talk to your group about what their expectations are. This can dictate a lot of how you role-play and how you make decisions. Does your group expect you to play your character within a certain set of boundaries (emotionally or mentally), or is this a no-limits, no holds-barred kind of group? This is something you can bring up with your group at the beginning of a session: "how much do you guys expect internal consistency on an emotional/knowledge level do you expect?" (or "is meta-gaming appropriate at this table?")
Remember that the goal of playing an RPG, most of the time, is not to win, but to tell a good story, whether you're role-playing your character or meta-gaming, remember this purpose.
A half-elf needs to roll a saving throw to avoid being paralyzed
Elves and half-elves in 5e share a common trait:
Fey Ancestry You have advantage on saving throws against being charmed, and magic can't put you to sleep
But this says nothing about paralysis, so in this case we are looking at a specific rule about elves and undead in the MM, and half-elves aren't included.
Best Answer
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...
– 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. :))