The last 500psi is for getting on the boat.
No its not. its a reserve to be used if needed. That "rule" was designed entirely to prevent idiot divers, most of whom have no concept or training of air planning at all from running out underwater. If its there then use it - you paid for it.
If you rent a tank, you do not want to run it empty. I will cost you alot returning an empty tank.
So leave 5-10 bar in it. Not a problem.
3-5ft seas really are nothing. You can breathe your reg, if not overweighted you can float nice and high away from it with your back to the swell. Ifs its big breaking surf firstly the boat is in a dull place and secondly a plastic tube wont help there either.