Just watched your vids. Just gonna post some thoughts here that may or may not be helpful.
-As you've already noticed, way too much DA (vs mk) Follow up with grabs more often.
-Vs both, you hardly used peanuts. Peanuts are good in both matchups. You can also mess around with cancelling it. Although everybody knows about it, you can still goof around quite a bit with canceling the peanut gun in numerous situations. I'll tell you some situational tricks on aim. Seriously though, peanuts are amazing.
-Z drop occasionally if you can. I just recently started doing this but its a good mixup.
-The most ironic thing for me was watching you try to follow up luigi with a grab right after I watched you follow up vs MK with DA all the time haha. Vs luigi you almost always want to follow up with DA. And spam fair vs luigi. I really don't think the matchup is too terrible. You did amazingly vs luigiking though considering playing what at first seems like an overwhelming matchup for maybe the first time.
-PUMMEL! You play peach, you know the importance of fresh moves and free precentage, haha. Also, dribble if you can against characters like MK and luigi. I'm obsessed with dribbling but I'm also a camper.
-Against MK, if you are recovering high, you want to do the side b kick if you anticipate mk going after you. It goes through all of his moves unless he predicts it and nados. When recovering low, you usually go for either the ledge or the stage. Stage is usually safest.
This is probably too much too fast/overwhelming, and I hope it didn't seem too negative because you're diddy is amazingly good considering it only being a week old. Very few people are as good/talented as you with diddy after that little amount of time. What matchups do you anticipate using diddy in?