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
If the few mA that the opamps pull are above the minimum current draw of your atx power supply, then technically yes. Most PC power supplies will introduce a lot of noise audible though.