-You can fast fall in tumble, yes. But if you're so high up, why NOT aerial (fair) first? Tumble leaves you with less options-- you'll have to tech or do a getup option or go to the ledge in a tumble. And I'm not too sure about this, but I think you're also more prone to jank windbox kills.

-sexy as hell. Remember that this is a MIXUP and not an optimal land, lol. Yes, it still works in Smash 4 the same way like it did in Brawl.

-Dair!!!!!!!! it has SO much less lag compared to previous games. It's also a combo starter at the spike sweetspot (e.g. into tipper usmash at some percents), but be careful since this can be teched. Sourspot dair (the hitboxes at the sides of the swing that send your opponent to the sides) is the safest part of dair to hit with, because it's generally bad to just land on top of your opponent. Also, it creates some breathing room for you to recover.

-land -> grab immediately. Maybe they're expecting a dair (lol, they shouldn't, because even though our dair is a BETTER landing option compared to in previous games, that doesn't make it GOOD.)

-If they're up behind you and you have time and you're sure it'll hit, maybe you can bair. Fair is weird to hit with when a person is coming towards you because the swing starts up high and ends low, but since bair starts low and ends high, it's more likely to hit.

-uhh... I'm running out of ideas. There's tons of crazy mixups like if your opponent shields under you a lot, you could go for a shieldbreaker instead of fastfall -> grab... there's the obvious "double jump away when they overcommit" (e.g. with a usmash or with a rising aerial)... I REALLY don't find Counter useful in this situation. Airdodge is generally the best option, especially since the Counter hitbox doesn't hit below Marth very well.