First up I must say I'm impressed how you are tackling the issue and posting up images along the way.
I've found 99% of the time the lower cover has rub marks, it just seems that old age deforms the plastic enough to cause rubbage - either that or previous attempts at replacing belts/doing work on the side of the engine have not been as careful as they should have been putting that cover back on. I note there is a philips head screw in one of the bolt holes by the water pump - it should have a regular 10mm head bolt like the rest of the shorter bolts.
Lower cover gasket isn't all that important, especially now that the cover at that age wont be straight and true and seal correctly anyhow.
To remove the tensioner I would attempt to put a pin through to lock it if the hole in the tensioner rod happens to line up with the rest of the tensioner body - if it is only ever so slightly out I often find rotating the engine a few turns is enough to push it back in perhaps 1 or 2mm. Once the tensioner is locked then undo the 14mm locking bolt on the tensioner and pull the belt off. I often remove the two 12mm bolts on the tensioner and remove it entirely before taking the belt off so it isn't so tight.
If you haven't been able to lock your tensioner with the pin (ie rod is too far out) and you have access to a shop vice then just pull the tensioner off and put it in the vice between blocks of wood and close it up enough to put a pin in it. Otherwise you can use a long threaded bolt (can't remember the pitch, but it's the same as the alternator tensioner adjustment long bolt) and thread it into a hole of the timing over above the air con condenser as shown here
