I might be a bit early to ask this but is there going to be ILs for this game? I just think it'll be good practice for me to do some ILs before the game.

i assume/hope that that's the case

Of course, they're already set up, just waiting out the 1-2 week grace period.

In the demo I noticed red coins that are already collected stay that way afterwards. If you beat the level again after you collected all 20 before it still says 20/20. So do you think red coins will be required for ILs? The game won’t count them again and the end screen will say you got it perfect 20/20 no matter what.

Once you collect something the first time, it will be ghosted out the second time you play, All flowers will be required for 100% ILs so yes.