First of all, Ethiopia is VERY hard, especially if you are new to the game. I know I probably wouldn't survive. When that is said, I'll try to answer your questions as good as I can.
1) The best way is through having a large military. You probably won't be able to sustain that though. Try to get an ally in the area. Also, try to better your relations, it should help if they get good enough, preferably above +100.
2) Most players just exterminate them. It's the easiest way.
3) AFAIK, no.
4) If you get Catholic rebels, you can accept their demand, which will change your state religion by force. You "risk" getting random conversions of your provinces if you are highly innovative and have, IIRC, humanist tolerance as one of your NIs.
5) First of all, you need to border a province owned by a western(ized) country. If that condition is met, you'll get a decision that shows what you need to do(or let you do it, if you already meet the rest of the requirements). The requirements are, IIRC, centralization at last -2(two steps towards centralized from the middle), innovative at least -2 and a ruler with certain skills, I think it is adm at least 5.
6) Tribal governments are the worst thing ever. Seriously, if you have it, you have a real problem. It's very hard to change too. I don't remember how, unfortunately.
7) If you cheat, you can. If not, send him to war.