hi Aurore,

I have a mamiya press (basically the same camera). I shouldnt worry about the pistol grip too much, you cant really hand hold these cameras without camera shake. just use a normal shutter release cable.

The lens should lock into position (it clicks when you twist it), sounds like it is worn. It still should be usable if you hold it whilst adjusting the aperature.

Light leeks are very common on these cameras, the rubber seals rot! I used thinish black felt to make new seals for the back which worked a treat. I have read that mamiya US will still supply original seals still.