Regarding the A/C changing modes on you after warm-up, I think there may be something else for which you should look. As most systems on M/B vehicles use the same type set-up I will tell you of something on my little '81 380SL. In the middle of the dash upper cover, between the middle outlet ducts there is a small vent like little sensor. Under the dash pad one will find a small duct that leads off to the right with a flimsy type little sleave. The purpose of this sleave is to send a good average of ambient cabin temperature to the sensor for temperature regulation. You may be able to remove the glove box (should not be too difficult) to look over at the sensor and see if the small sleave is in place. If not, reconnect or if it is badly deteriorated you can just put on some clear plastic tubing and lead it over to just under the glove box somewhere. This will enable the environmental control to function properly (hopefully). Don't get flustered, everything is an education.