No, at the present state-of-the-art, 'un-baking the cake' isn't practically achievable except in special cases like a stereo recording where ONLY one instrument being panned centre etc.
However, I would be very surprised if the military aren't interested in software that can isolate one voice from a babble. If that is successful, stand by for a spin-off.
No it is not possible given the current state of art. Therefore recording engineers distribute microphones to have separate tracks for the instruments or instrument groups (so they can change the balance) and therefore special recordings as music minus one are required with filtered soloist voice to produce training material with full tutti voices and ...
If you want three notes to fit in a quarter-note beat they should be triplet 8ths, not triplet 16ths. A 16ths triplet will fill half a beat.
I'm also wondering if you're confusing a triplet with a beamed group of three notes.
There shouldn't be any way to make three sixteenth notes into a triplet, since they can be written normally. Tuplets, in general, are used when composers want to write a division of the beat that cannot be expressed in powers of two.
For example, you could make four sixteenth notes into a triplet, and the three notes played would be played in the same time ...
Short answer: Yes. Software can work, but only the right software. Your generic guitar tuner app won't do it properly because it simply tunes to the theoretical frequencies of flat equal temperament with no "stretch", which is really important for a piano to actually sound in tune (particularly in the treble and bass). As a piano tuner I also don't trust ...
I don't think you can. At least the command line options don't mention anything about playback: https://github.com/ReaTeam/Doc/blob/master/REAPER-CLI.md
But maybe that's not really what you want, i.e. why do you want to play a reaper project and not a sound file ?