I can answer a few of these for PP.
DL64 is Marth's worst neutral and Falco's best. Marth cannot fsmash underneath platforms, and his utilt/uair cover far less of the platform than other stages. the larger stage gives falco more breathing room, and subsequently makes it more difficult for marth to get in on you. marth gets a pseudo chain grab, so be careful about center stage.
Vs fox's shield pressure, you need to learn what you can do when. vs high nairs, you can do anything you want, but vs delayed aerials, you just have to wait it out, roll away, or hold down and mash L+A or B if you're at low percentage.
Vs badly spaced nair/bair at low percentage, always, always CC shine. If you can't, and aren't in the position to be knocked off stage, DI down and away (teching, if applicable) to make it more difficult for Fox to follow stuff up.
Vs well-spaced aerials you're just going to have to DI away. perfectly spaced nair and bair cannot be punished. fox is faster and hes just outside of the range for any of your quick moves.