Those would be fantastic buffs. Yet still, the ability to shorthop/B-reverse is pretty crucial. Mobility and unpredictability is key in this game; even with lower endlag the move would be incredibly easy to bait out and punish. Just why does it makes sense for D3 to only be able to jump HIGHER when he pulls out a rocket hammer, and not low? Dumb.
I do see your point about the movement but I don't think the whole baiting out and punish bit makes sense. Not the way I see the way jet hammer would work with less endlag.
The thing is, jet hammer would still leave you wide open to projectiles and all that stuff even with a short hop and b reverse and direction change in midair. In neutral, all you're really doing - even with all the buffs both you and I mentioned - is taking away your ability to shield and airdodge. The hitbox doesn't cover a lot in front of dedede and the hitbox is out for two frames iirc. In neutral, all jet hammer is is a scary sound and a bluff that's either going to earn you 30% or kill you for your troubles, or theyre going to ignore you/throw projectiles at you until you give up trying to land something that isn't meant to be. Running around with jet hammer still wouldn't be a good neutral tool. A jet hammer with less endlag means you can just pull it out on someones shield and space it half charged and get away with it, but you still wouldn't be able to run around and play footsies with someone even if you got added movement abilities.
Now on the
advantage side of things, jet hammer becomes one of the scariest moves in dedede's arsenal. While the opponent is on the ledge and you pull out a jet hammer that can kill you at 40 and all they can do is run away since you can't punish him effectively for whiffing it and you can't shield it since you waited too long, the opponent starts playing mindgames with themselves, which they currently don't since all they have to do is escape, then they can punish as hard as they want, at least in the case of full charge. Sure, shorthopping would be cool but its not really a huge deal at the ledge since you're not going to get far with it in neutral and you can just space it to cover getup attack anyway.
Extra mobility would be nice and all but it definitely shouldn't be prioritized over less endlag since less endlag means scarier ledge game as well as offstage edgeguarding and the mobility wouldn't make a lot of difference anyway. And it would probably take sakurai a bit more coding to add a shorthop in than to decrease the endlag on jet hammer.