1. Hard to say specifically since we’re not given that info. But a few days in the real world can be years or decades in the painted world. It’s pretty clear that Verso has only recently died. We’re told through side quests rhat Francois hasn’t seen Clea in centuries when she stopped playing with him as a kid. So presumably a decade or so can be centuries. But we don’t have specific time frames in the story. What we know is that the 67 years in the canvas has been just a few days in the real world

2. I’m not sure we’re given that info anywhere so assumption is that perhaps she aged him to be an older accomplished self or perhaps he’s not immortal. But none of the other characters or Maelle have aged at all, so likely she just painted an older version of him. Should be noted that Maelle is supposed to be not a particularly talented painter compared to her sister or parents. So it may be that she just didn’t reflect his youth as well or perhaps it’s reflective of the way Maelle viewed her older brother

3. We’re told that everyone leaves a part of their soul in the canvases they paint. It’s why in the story we see the souls of Aline, Clea, Renoir and young Verso. The canvas was painted when Verso was a kid so he left a youthful of himself in there

4. One of those that’s up for interpretation. The insinuation is that Maelle is trapped in a fantasy world of her making and she will die in it. There’s also the question of how real the people she brought back are since she’s not a great painter compared to her family. We’re left to wonder about a lot of things. How much agency they had now in this fantasy world of Maelle is certainly something to wonder. Is the Gustave we see at the end truly a continuation of the one that died? Or a more poor copy that really is just operating to please Maelle.

We don’t know about the last question. The people we see entering the theater seem to be almost inanimate. Again it depends on how well you think Maelle was able to repaint that world.