The lever cap problem is illustrated by the 2 pictures where it can be seen that during mid closing of the lever, the tension is a lot stronger than when the lever is closed.
During the mid closing position, the distance measured 6.5 mm, and when the lever was closed it measures 5.2 mm
I solved the problem by rounding the corner with a file. So now the mid closing distance is more like the 5.2 mm, I didn't do a second measurement, but it feels right so it is probably OK.
The frog had a little rust all the way down, where the blade is supposed to rest. I took a file and gave the frog a few swipes with it to remove the rust. Much to my surprise, the front of the frog was so poorly machined that the blade wouldn't even touch the end where it should. I filed some more, and ended up with a flat front after some time.
To make the frog fit the sole, I applied some grinding paste and tried to see if that would do the trick. It wasn't quite enough, so I had to remove a few burrs with the file. Then back to paste grinding.
I had found that the frog couldn't be moved far enough forward to make a really tight mouth, and I suspected that the washers for the frog holding screws were the cause. I found out that adding a washer behind the slotted piece of metal also enabled the frog to be able to move as I wanted it too.
The sole needed a little flattening, but nothing serious.
All that is left now is to sharpen the blade and test the plane.
Lever cap showing the mid closing lever problem.
Distance with lever closed.
The finished plane.