Aww, sorry to hear about Max!! may he swim in peace!
As for Guillermo, some Betta's are just too aggressive. However, there are things you can do to still make a split tank. I double up on my mesh when a betta is too aggressive on one side so they can't see each other as much. I have it so that I'm basically moving one of the mesh off just a tad so that most of the holes in it are covered up, water still flows through so that's not too much a problem but it helps because they won't be able to see each other as much as thus not being so preoccupied with each other. so that's something you can do.
I also had a Lee's Divider for the 10 gallon so that's nice and then I siliconed another piece of mesh on one side and now Mercury can't see anyone because he's one of my more aggressive boys as well
That's cute about Ash, yeah my boy's recently got over that a little more. They went through two spurts of being "Sir Zoomsalot's" first when they were about 3-4 weeks old in my care and then again at 7-8 weeks old so it seems to be a monthly thing for my boys haha. Hopefully you can get some pictures soon though, I'd love to see how he's turning out!!