With 1 & 4 I have been down this route before. Seen this shadowing behaviour quite regularly in Lembeh and have been advised the "shadower" is either Carangoides coeruleopinnatus or C. armatus. Personally, I think the latter. They always in my experience follow various types of Puffer.
The last one is a blenny - Petroscirtes mitratus
No. 2 is some form of Conger - genus Ariosoma maybe
I was in your position and changed mine for Nauticam clamps. Very pleased with them after a year. I also have some I-DAS stuff (not clamps). Their kit is quite cheap but not as well finished. After every trip I have to pickle all the I-Das gear in vinegar to get the white crud off.
Are there any further reports and experiences of using the Canon 5DS and/or the 5DSR underwater please? I am planning some changes to my gear and this is a possibility but there is a dearth of information about its performance underwater.
I have that set-up but mine has the o-ring. It grips well but, like you, I am still nervous. So I go along Elmer's route using elasticated cord attached to frequently changed, stout rubber bands round the lens (a cable tie sounds better). The lanyard also attaches to the adapter so that if the whole lot comes off and takes a dive or more likely gets knocked off, it just hangs under the camera. I don't like bits of lanyard around but in fact I never notice it once in the water and the peace of mind is well worth it.
Drilling a hole sounds a good idea if you are handy at that sort of thing but the mount seems pretty crappy material to me so take care.