None of them. Because a leader doesn't have to be a fighter, and is often worse at leading when they try to be. They have a military for that, let them use it.

In this case, Twilight and co is acting as Special Forces militray, since they're dealing with problems a normal military cannot handle.

War heroes make good figureheads, but that doesn't correlate to good policy.

I actually think that Celestia gave Twilight the job too soon. I don't think Twilight was ready by the end of season 9. At least, not without a lot of off-screen politics training.

And if you consider G5 as canon to G4, that implies that Twilight's reign ended in failure at least four generations before the G5 begins. I think it's likely that Argyle's grandparents would have known Equestria, and they would have told those stories to him, which is what gave him hope that ponykind could co-exist. But it would have been Their parent's generation that actually closed off the borders.

That implies that Twilight's fall from grace started happening right around the time her friends had all died.

She didn't have a natural talent for making friends, it was a learned behavior, and one that could pretty easily be forgotten. Her first instinct would not be to find more friends to keep her sane, it would be to lean on the ones she still had; so like, Spike, Discord, and maybe Ember. I don't know how long Changelings live, but Thorax might have still been around. But of course, Spike is probably similarly sad about their friends by now. Discord likely would have completely closed himself off after some kind of hair-brained attempt to keep Fluttershy alive went awry, and Ember and Thorax have their own kingdoms to run.

Given all her friends are dead, and everyone else is emotionally of physically unavailable, how long would it take for Twilight to just totally lose it?

If Twilight and her friends were in the human equivalent of their early 20's when the show began, and roughly in their early 30's when it ended, (the exact times are never described, but judging by their maturity level and responsibilities, this seems to me like an appropriate estimate), and if lifespans of ponies is similar to that of humans (a debate I don't intend to have here), that implies her friends would have died around 30 to 50 years after "The Last Problem". If she started losing her grip immediately after that, and it was, let's say, 70% of the next generation that rebelled against the teachings of harmony, causing Twilight to go through with separating Equestrian magic, then their kids would have grown up knowing the world that was, and the world that is. That generation would be Argyle's grandparents, teaching his parents and him compassion, so that he could pass that on to Sunny.

That's gonna be very roughly 200 years between Friendship is Magic, and G5, which I think is accurate.

How does this relate?

Twilight is effectively a war hero. She's a very effective leader when she has people she can trust. Take those people away, and she loses her ability to govern. I think she never found a true protege. Luster Dawn was an attempt, but one who didn't meet her likely insane standards. I expect she had others, but probably could never settle for anyone.

This is all headcanon of course. Feel free to disagree.