Hypertrophy (but not cardio, surprisingly) exercises help me sleep, so I'd really like to do some of it every day. If I manage to build a routine where I have at least 48h of rest for that particular set of muscles (say, biceps and lower arms from the curls), is it ok? Or is the rest itself not really "isolated", in the sense that I'll be overloading myself regardless?
I guess the better way to ask this is the following: say you build an absurd weekly plan of 1 day curls and 6 days squats. Are you only overloading your legs and preventing them from growing but not your biceps, or is it a global condition where your curls will also not be as effective?
for isolation exercises you could make an everyday routine that works for you, it wouldnt be optimal for gainz but if working out every day is more important to you it'd be ok to do.
your central nervous system gets fried by heavy compound exercises (eg squats) and trying to do them multiple times per week at high intensities is almost always a really bad idea and does cause more global issues where you won't be able to recover to perform your other movements properly.
you can always always do abs. every time I want to do a session that's not in my program I do extra abs.