first vid comments: if they're above you and you have a banana right next to you like you did when you jumped up probably to uair the MK and he dair'd you, just upthrow or glide toss upthrow at him to an an one of the uair, usmash, or utilt attacks. the ledgehop>dair doesn't really work too well for most people so i don't know if that's your thing, but for most people with diddy, it's not too great. lastly, barrel spike is good. if theyre recovering and you just tap away, and start charging the barrel, you can get some good kills.
second vid: SH> banana pull out is safer. you avoid some attacks you don't expect on accident just by pulling out bananas. i wouldn't use dair to attack from above unless you're spiking... from above i will usually dthrow or zdrop a nana, do an airdodge>shieldgrab (kind of what i meant at the end of the video, when he did a FF airdodge>fsmash kill. that was good, i just tend to shield grab because people like to do lagless attacks from below to prevent what you did.), avoid them, or fast fall a fair.
third vid: nothing except your jumping after forward b. i don't know exactly what it is but i like to do it before. the flip sends you back a little before you go forward, so it can kill you if done next to the boundary. i could see why you would do this to prevent kung fu SD, but if you grow out of doing that, i would do it the other way around. i also see how it prevents close to edge... edgeguarding, but also if you jump first you won't need to do an up b when in most cases you would, because >b sends you a little up, so you can DI back instead of using upb, in order to give yourself the ability to airdodge then upb, instead of being stuck airdodging then doing a low uncharged upb... resulting in death. now that i talk about it... there are pros and cons so it's kind of your judgement. what do you think?
. Critiques would be good I guess if you want. Keep in mind I just started diddy though so don't be too harsh 
