I cleared his name and he said he would be in the city if i need him to steal something for me. I am at the part in the main quest where i tortured and burned the 3 heretics and they confessed.
I assume i have to find thintan hand have him steal the letters that they mentioned. Problem is i can not find him. I checked some guides and they say he should be at the spring in the city, but he is not there. Since i cleared his name a long time ago i have no earlier save.

i checked:
-his spot in the wasteland
-both inns
-all wells/market places
-grand dukes gazebo
-ran around both city parts for a while

Can not find him anywhere. Do i have to talk to someone for him to spawn? Is there a step between the letter confession and beeing able to go after the letters/the last heretic?

I can not enter the dungeon under the church to go there myself as the door is locked and i can not find a key

He should be near the fountain in Northwest Alvaron.

EDIT: Oops, I just realized you are not that far yet. Tinthan makes several moves around the world map, the first should be to West Glatzburg (near the fountain west of the smithery). Then he should move to the Grand Duke's Gazebo. And only after that he should move to Alvaron. But players do have troubles finding him for some reasons, so if he isn't at any of the first two locations mentioned above you could try to add him to your current location through the console:
- press ~ (next to 1)
- type \addbeing_afriend Tinthan -1 -1
- press Enter. He should appear at your position then. You will get the key once you will really have to go in there.