It is very easy to open. There are 2 screws one at each corner on the top, two more on the underside by the spool. Once those are out, take off the dot of leatherete on the lever and unscrew the large center screw. The lever should pick up but be carefull to not lift the winder or you will lose the spring tension. No problem putting it back as you will figire it out sooner or later that it catches the matal plate that sits on the spindle.
Whe taking the top off, watch for the little slip of metal that covers the hole on the lever side. That has to slip into the slot, just as you will need to do when reassembling it. I put the insert in multi froame more so it will retract the slip of metal, it makes it easier to remove the over.
Your problem soounds like the small lever that hits the edge of the shell is not engaging properly. That will be on the counter side near the top.
Put the insert with the lever reinstalled into the shell loaded with a test roll of film, just the paper backing is enough and try cocking n firing it to see how everything works together.
BTW while it is open install new seals before reassembly, it makes it so much easier to do.