1- For testing the AC you will need special tools, it is probably beste to go to a AC worshop. They have the testing equipment. Last year I had an faulty pressure switch, it took them 10 mintutes to find it.

2- If you want to ajust the tension of the belt you first have to loosen the big bold in the center of the (round)rubber tensioner. Just loosen it otherwise the tensioner may drop out of the adapter plate. I'm not sure witch way to turn the hollow nut, but if the treaded rod goes down the tension will increase.
On my 126 it is possible to quick release the tension by removing the lower alternator bold and swivel the alternator up.

3- Starters are generally easy to remove. Don't forget to disconnect the battery first!. It will be easyer it you can acces it from below. Don't drop it on your head, these things are heavy.