1. You should promote the highest officers first, usually. This pays more off.
2. The very value of renown is in that you can buy upgrades with it. The more you sink the more renown you get. You have to save up for the upgrades. You get also renown for reaching and finishing the patrol and returning to base (rather than click "return to base")
3. Don't know, I have always played at 100%
4. I think it's better to save the renown for ship upgrades. But it might be a good idea to recruit a few really good (500 renown) officers to start with. At least that's what I do. Spend the first renown on the officers.