climb from celeste. it'd help a lot with platforming, especially pre-booster 0.8/2.0. they could also change the last cave to have certain parts requiring some climbing. plus, it'd be able to let you grapple onto ledges with the tow rope!
switch version or the original freeware. if you don't have a switch but want more stuff than the freeware, go with cave story+ on steam. it's not as good, but it's good enough.
yes! it's portable, has more soundtracks (ridiculon and famitracks) and also controls better with the joycons/pro controller/gamecube controller (i use gamecube controller lol)
i'd go with the bubbler. the fact you can hold that many shots without them actually firing, and yet still have them do contact damage at level 3 is kinda op. level 2 is good, and level 1 has to be the worst.
in my opinion, i REALLY hate the CS+ soundtrack. i prefer to stick with original/famitracks.
also the new textures are just... ok in my opinion? i don't think they're good or bad, to be honest.