1) No, flash is simply no longer getting support from the company that developed it, which means bugs that are still in when support drops will be in forever. Nothing will change on the user's end. 2) This game is not ancient, that would be its first predecessor, which is not the same game. 3) Not a lot has changed, but to say nothing has is completely false. There are new voiced lines, new clothing assets, new background assets, etc.
Not much in the customization category, but new backgrounds and voiced lines were added as well on top of lines from the previous two games, though no previous backgrounds as far as I can see.
Breast size can be toggled in this game if that's what you mean.
You can't, either default size or smaller.
No, similar, but not the same.
This game didn't exist then, but its predecessors did.
There is a toggle for that.