I'm in Ester City and I'm about to enter the tower. Is it possible to go finish the game now and play the RPG minigame afterwards (by loading the last saved game or something similar)? I'd like to do that, but I'm also a bit worried that this might be the last place to play the minigame. And I wouldn't want to play the entire game again just for that. (You can get locked out of Sam's abilities like this earlier in the game, for example.)

Ok, so I will answer this myself and leave it here for people who might stumble upon this in the future. Once you go into chapter 8, you cannot play Earth Born any more (ever - you need to start a new game). If you want to play it after completing the game, you need to have a backup of save from chapter 7 (that's what I did, because I had a suspicion that it would be like this).

I can't say this is great game design... But I've heard that the devs are working on chapter select, so hopefully...