The third option is the closest.
Pogba is obviously a world class talent. He has the potential to be one of the best midfielders of all time. However one of the most important things needed to reach the top level is consistency, and that is something he has shown absolutely none of. He can be one of the best players on the planet for a month or so, and then the next month he'll be decidedly poor with the occasional match thrown in where he is absolutely atrocious.
Part of it is obviously the situation surrounding him. The manager blatantly mismanaging our entire team, both on the field and off it, creating a situation where literally not a single player in our entire squad (bar our goalkeeper) was able to show any consistency at all. Poor tactics, poor formations, poor player selection, etc. That hurts consistency both directly and indirectly (by hurting confidence and causing players to be unhappy; doesn't matter what job you are in but if you are unconfident and unhappy your performances will suffer). Quite a few players around him not being good enough to pull their weight also hurts. Him being relied on to pretty much single-handedly control the team around and create the goal scoring chances himself doesn't help (almost all top teams will share that duty amongst at least two players, often more).
However, part of it is Pogba himself. Even with those excuses that I listed above which would certainly explain some inconsistency, he has taken it too far at times. There are definite questions about his mental strength, as when he really puts his mind to it he can do pretty much anything, but too often when things aren't going well he does end up sulking with an almost 'woe-is-me' attitude on the pitch. Instead of rolling up his socks and working his arse off for the team (which he is quite capable of doing when he really wants to), he just drifts around and plays far too individualistically. He doesn't show the hunger and drive to minimise that inconsistency.
Some people seem to focus almost entirely on the good aspects of him and ignore the negatives, showing a definite bias towards him. Equally, some seem to focus entirely on the negative and ignore all the good stuff he has done, and those people are showing an equal amount of bias. The media are often guilty of this, as are you with your claim of him being awful for the 2.5 years under Mourinho. He has been inconsistent as all hell, and has had many periods of poor form during that time. But he has had just as many, if not more, good periods where he has been quite obviously a class above pretty much anyone else in our team. The sad thing is that even with that inconsistency he has been arguably our best outfield player during his time with Mourinho. Not because he has been amazing, because he certainly hasn't. But at least he has had fantastic periods. Nobody else on the team has even had that. Pretty much everybody else has been just as inconsistent, but their highs haven't been as good.
I have no doubt that with a better team and better tactics around him Pogba will improve his consistency. Hopefully we will see that under Ole. The big question mark though is whether he will improve his consistency enough to truly take that step up, both individually and to drive the team. Hopefully he can, but like I said that is the question mark that is hanging over him.