1. Raging Strikes (+damage) and Quelling Strikes (reduced enmity) are both useful abilities from archer for black mage.

2. There are pros and cons to both methods. I'd personally recommend sticking with a single character, as you get an EXP boost to combat classes which are below the level of your highest. I.E., as a level 50 GLD/PAL, my level 20-some-odd CNJ gets 50% or more bonus experience. The main downside is not being able to reap the sweet benefits of doing the story or side quests again, but with FATEs being such good experience, I don't see the harm in this.

3. Just spend them. With very few exceptions they should go into your class' primary stat. In your case, INT.

4. Your basic class and the accompanying job are intertwined. That is to say, once you convert from THM to BLM, your THM doesn't stop leveling up. You still gain core class abilities, in addition to Job class abilities from quests.