jcclink

Guitar picks work great.  They come in several thicknesses, so buy a selection.  Like you say the main o-ring is a tight fit.  It takes a little practice removing & re-installing.  Place it flat into the groove with a little finger pressure.  If you work it parallel to the groove it will elongate slightly & won't fit properly (appears too long).

 

On a trip I only remove the o-ring if its really necessary to properly clean it.  Most of the time I can remove minor debris with it in place.  Try to keep the area between the o-ring & groove clean.  Something like the corner of a business card works for this.  You can flick out small particles with it.  I usually can do a long dive trip without removing the o-ring but do remove it afterwards for a good cleaning & complete re-greasing

 

I suggest buying a complete re-build kit for the housing.  It includes ALL the o-rings & new stainless c-clips that go on the button shafts.  Also 2 strap wrenches (plastic handle with about 20-25mm wide rubber strap).  Sometimes its really hard to unscrew the port.  Nexus is a great housing.  Not many of us use one.

How far can you lag behind your dive group before you're considered a solo diver?  If you're a photographer you will be a solo diver at some point.

If you use a vacuum leak detector, rotate all the knobs/levers & push all the buttons several times.  It's possible to have no leaks in static condition but housing could leak with a dynamic (move all controls) test.  Ditto on hydrophone cable - it serves no useful purpose u/w, unless you like to sound of bubbles or shrimp.

Over 35 yrs designing u/w electronic housings & using several u/w still & video systems, I've always left o-rings in place with housing closed.  No issues.  O-rings have been buna-n (the black ones).  Keep clean & a little silicone grease when required.  If I bought something with colored o-rings (like S&S strobes) I changed their blue o-rings to buna.  Your main concern is eleiminating dust or debris, not compression of o-ring.  The more you play with the seals the more likely a potential problem.