If he's consistently shielding right as he lands, then you should be able to get some free grabs out of it, until he realizes what he's doing. Honestly, the aerial mobility of DK while he's Up-Bing is pretty good, but it's really not the toughest thing in the world to chase and capitalize on, especially once he's committed to recovering on stage.
As for practicing tether cancel into Screw Attack, get a good feel for sweet-spotting the ledge out of it. We don't get a second chance to tether in the next update, so it'll be very important for you to space well and get it right the first time.
Talking about Mewtwo is no fun. The thing to keep in mind is that he never really loses the neutral game. As it stands right now, he can always teleport into an attack, which means you can very well be dealing with a constantly aggressive Mewtwo. If you are dealing with a player who loves to constantly teleport, the first thing to do is study the distance of TP. It boils down to just being better and more clever with your spacing, and being able to stuff his approach with a hitbox that stays out for a while (usually nair, but I've sniped their TP with ice up-smash before).
It's far more easy to deal with Mewtwo when you're using Ice beam, just because he's basically Samus in regard to his gravity and weight. Get very good at powershielding (not hard at all), and you begin to eliminate his ability to hit-confirm off of shadow ball. I tend to never go for grabs, just because if you whiff, he can grab you, and his grabs hurt us more than ours hurt him. A lot of my neutral game has turned into MCing from mid-range to even short range, followed up immediately with a Zair, which will cover the option if he powershields or side-Bs my missile. If he's in shield from the missed PS, or just to avoid the Zair, a lot of times I can get away with a dash grab Dthrow to upsmash.
When it comes to his recovery, I tend to respect it a lot, and just establish positional control. Missile spam works fine just to fill the gaps (and the time), and can sometimes force him to choose an option that you can easily cover.
Kirby is just an annoying ****ing prick. I've got this really campy Kirby at my locals that can be really frustrating to play with. Basically, all he does is crouch under my stuff, and then punishes hard as hell, because game design. Luckily, ice mode can kind of **** him up. My game plan went from DAFTing him to just going for really late fairs our of my SH. Swat the little bugger away and charge up my beam of doom. I started treating it more and more like playing against Puff in Melee. Lots of respect for the characters spacing and moves. Just hit him once or twice, as hard as you can, and knock him away to charge shot. Take pot shots as much as you want, he can't do much about it, and he can't really get to you in time to punish your charge time when he's at mid-percents.
Of course, once I started doing better in the neutral against this Kirby main, he started playing super cheesy and tried circle camping me on the platforms. Patience is a virtue, my friend. Homing missiles started becoming a smart option, distracting him enough to go to top platform, where I'd be already heading to throw an Up-air to cover, or he'd land on an edge of a platform where I could easily punish with ice up-smash.