Callouses over time will form depending on how much you play. You can also just slide your fingers up and down the strings while pressing against the fretboard. This works well if you have roundwound strings (the usual type of strings that squeak when you move your fingers over them, most non-nylon string acoustics use these strings) because there is more texture. Once formed, they will last a long time, especially if you play. Over time they will start to weaken, but we're talking weeks of not playing.
Steel string acoustics differ a lot from electrics in many ways. Because I think you are asking about fretboard action, and not about the other stuffs, I will focus on that. The "action" on an acoustic will be higher than an electric. This means that the strings are strung slightly higher off the fretboard, making it harder to press down. Starting on an acoustic will build finger strength so switching to playing an electric will not be an issue. All the same muscles are used and your callouses will be the same as well. It is similar with a twelve string. The difference is that you have 2 strings for every note. When you press down to play a note, you will press two strings. These strings are tuned to play the same note either in unison, or in octaves. What results is a "chorus" effect. Again, if you learn on a 6 string, a 12 won't require much overt change, just subtleties.
A bass is a guitar. The difference is the fretboard is longer, the frets are bigger and the strings are bigger. This allows the bass to play the octaves below a "regular" guitar. Again, it won't require a great amount of re-learning, but rather just re-adapting to the new setup. Your callouses will be pretty much the same, they are versatile callouses. Though, if you finger pick the bass, it tends to tear up your fingers pretty well, so you will eventually form a callous for that.
Is the gradual sharpness audible, or are you noticing it because you are watching the tuner? If it's not audible, I wouldn't worry about it. If it is, and it's really obvious, then something is funky. It could be that the strings are vibrating against something, or the neck is shifting but that seems strange. I doubt it is your microphone. How many cents does it go sharp?
Depending on the capo, it is merely a matter of preference. Just make sure it is covering all the strings and that it clamps with enough force to ring a clear note.
You don't have to use a pick. Do whatever is comfortable for you and whatever fits your style. Though, I would recommend trying out as many things as you can when you are just starting. That way you know what suits you best. To learn to fingerpick, you've got to finger pick. You can also use a hybrid technique which allows you to hold the pick between your index finger and thumb while picking with you middle, ring and pinky fingers. But that's a little advanced.
When I started learning, I was teaching myself. It was fine, but I decided to start taking lessons a few months later and my abilities improved much more rapidly. I was amazed. So, for me, it was positive, but that doesn't mean that it will be positive for everyone. I think a lot of it has to do with personal preference and willingness to learn and practice as well as the person you are learning from. I would, if I were you, do some trial lessons before you commit to signing up for full lessons. That said, there are a lot of fantastic guitarists, present and past, who taught themselves.
I have yet to have any ladies throw themselves at me because of my guitar skills. But that's just me.