What you seem to want to do is definitely doable. "Writing an entire app" sounds a lot harder than it is. You don't start out at 0, but with a fully working app, with all hooks that just doesn't do a lot yet. So it's really not that big a deal :-) In most cases you can just mod an existing app but I personally prefer to statr from scratch rather than digging thorugh someone else's code ;)
It absolutely can, and it doesn't even have to be that complex. A single program change message could do if you write the app with full support of all the possible pedalboards you have.
Short answer: Lots. Long answer: 90 MIPS -> 90,000,000 instructions per second -> 90,000 instructions @ 1kHz. Semi-random very pessismistic guess per ADSR is 1000 instructions -> 90 ADSRs.
Any particular reason you sent that as PM as well? Tiny traces, tiny spacing, >400 holes - not very diy friendly, and certainly not worth the effort, when you can buy one for <10eur.
if (PORTCbits.RC5) { // pin is high do something } else { // pin is low do something else } Remember to set the pin directions before using them as in/outputs.
Yes, that's possible (depending on the app you want to use it might mean some coding [for the mbCV for instance the amount of coding = 0]), the triggers are 5V