1. I believe a main character is one you play the most often.
2. This makes an alternate character any others that I might have.
3. I believe that it helps to do this. The important thing is to communicate with each other OOCly. Before, during, after. I personally wouldn't want to commit to anything long-term if the other partner isn't seeing it that way as well.
4. No. We discussed the relationship before hand: how much time would be committed to playing that relation, or what to do in case of long absences (they're not always planned--real life happens).
5. I hadn't really thought about it.
6. Same here.
7. I tend to stick to one main at a time. If I find myself becoming more and more active on an alt, I switch which one I name my main.
8. I play whichever appeals to me at the time. My main tends to be my roleplay character, which is almost always the reason why it's my main; they're the most fun.
I have made alts as secondary or throw-away characters. They get brought in as the story needs them.
I have made alts to try other classes (this won't be an issue here), and if I end up loving the class they get an eerily similar look and name to the main. Other's won't need to guess if it's me because I make them the same.
I also make alts that are my 'don't bother me' characters. I've yet to roleplay on them, and the few who I do trust enough to tell my identity to know not to bother me unless it's important. On any other alt, I make myself known, and will switch when asked and it suits me. It shouldn't hurt anyone to be told 'no, I have other plans'. Again, discussing how much time is expected of each other before-hand helps here.