^^^ You're mistaking floaty characters with light characters. Ganon is floatyish, and I don't think Kirby is floaty.
Sheik is also floaty, but IC's are really good against her.
A way around the dthrow dair on some characters is to have nana jump out more with dair, so it'll hit them towards you as opposed to away. Even if they intend to smash DI away (since they probably won't expect you to do it backwards), that will still put them in range of another grab.
You can also try dsmash during dthrow's animation as it adds more stun to it after they're thrown, which might allow you to regrab. Other than that, just go for a lot of baiting and try to land wd smash attacks on them. Try to stay below them as uair and wd utilt (a personal favorite of mine) are really good.
for edgeguarding Marth, there's not much you can do other than the lightshield edgehog trick, to ledgehop nair or get up and grab him (you'll probably grab him facing the stage so do uthrow reverse fsmash or dthrow bair). Depending on the stage it might be better to go for an usmash out of grab kill rather than fsmash or dsmash.
Against Doc, you would probably be better off killing him from the side since his recovery sucks. Don't give him space to throw pills at you, and just do baiting and switch things up (maybe throw out desynched ice blocks or blizzards). some safe approaches are wd jab/ftilt.