I'm more inclined to think DH requires a rather unique mindset to work in FFAs... Often when I see people complaining about a character's FFA viability they either don't understand how the character is supposed to function in FFAs (like a Palutena that wanted to play "spam smash attacks" like a heavy instead of turtling and following up on other's attacks with Uair and Usmash) or expected instant/extremely high results. (Which would be quite a few Samus players.)
I suspect that to succeed in FFAs with DH you need to utilize his Fsmash more. Namely charging it to increase the range. Get close, get out a gunman, and charge that Fsmash and pray somebody falls into the last hit.
I
really don't think Pit has to fear Dorf offstage. Namely that he doesn't have a solid answer to the armor on Fspecial or a way to contend with arrows. His aerial mobility is too low for Pit to really think twice about using them.
...Well, pretty sure Flame Choke beats Upperdash. So don't get offstage when you're a stock down.
Pretty confident Pit has the tools required to keep it from slipping that far though.