Before you start playing with adjsuting the screen or anything.. take the lens off the camera.. now look at a white backgound in your VF.. how sharp are the lines n grainof the screen? If it;s out of focus you need a diopter!
To adjsut lenses for infinity... You need to use a colimeter not a target for infinity focusing. There are ways to make shift a colimeter using another camera and a tele lens. Look it up on the net?
Also if you are going to mess with the spanner screws (large flat head with 2 holes) you should make a small reference scratch on each head to teh body so if you have to go back you can undo your screw ups.
Also when checking focus film to lens you have to use a ground glass on the film plane to compare the VF to it. Doing it by eye is playing a dangerous game.
Your foam seals wil not effect the focusing as much as you think.. the difference is so small nad the way it locks it will force it to the minumum thickness or it will not lock!