1. how do you cast spells on your teammates? I have heal and I can't seem to heal the other person. Am I missing something?

2. How do you raise spirit resistance? I noticed intelligence will raise it (and constitution raises poison resistance). Will you get items/gear later on to increase these to a significant amount? I'm currently playing on hardcore and the things that hit for spirit damage hit like a truck.

3. When I reload the game goes completely silent often. Is there a fix to this, or am I doing something wrong?

4. What's the point of teachers? I came across an IMP willing to teach me skills I can already learn such as lockpicking. But it says level 10, does this mean learning them means I can put up to 10 skill points into an ability instead of just 5?

An 'Individual' spell only effects the character casting it; 'Party' spells apply to allies, as well. There are party based elemental resistance/reflection spells, but no healing spells.
I had both my characters learn level 2 of the individual Healing spell when I got the book that teaches you that, but didn't bother with the spells to cure/remove poison, disease and curse.

You will get items and equipment bonuses that can raise spiritual resistance, but it is primarily from intelligence (bonuses for other resistance types are more common). I didn't really worry about it, though; it is mostly just an issue early in the game.

Does the music start up again after a few seconds?

Yes, trainers can increase the level cap of skills you already know, in addition to teaching you new skills. It doesn't indicate your current level cap for the skills, though, when you are talking to the trainer, which would have been helpful.

Of course, I don't know why the individual category didn't click in my head.

Thanks for the info :)