#1
This might be a strange way to approach effects, but is there software out there that can add decent-sounding effects to recordings? I'd like to add some reverb and other spacey stuff to my vocals and maybe the guitar (if it sounds good). I'd like to just be able to edit the existing wav file directly, like sound recorder but better.
#3
Not if you want to apply separate effects to each instrument. The .wav will be just a stereo file, not individual tracks, so anything you apply will affect all the instruments equally.
#4
if the track is already mixed down and includes all the instruments, no you cannot do that. if you have each of the original tracks that you want to apply effects to, that is very, very possible.

any decent DAW, and even some not decent ones, allow you to apply effects to your recordings. you would put your vocals .wav file on one track and guitar on another. add your effect to each track individually and it will only apply to that track.
#5
I should have clarified. Everything is recorded on separate tracks and stored in wav format, so I can isolate the instruments.

jof1029, what is a DAW?