Trapeze?
I dunno, I've never used one. All the deco I've done was either on a line or on an SMB... With a large group I suppose it would be helpfull to spread out the divers on the last stop (we just use our reels as john-lines if it gets too crowded on a line - keep it simple).
You could attach one end of the trapeze bar to the rig I posted above and add a second ball to support the 'down-current' end of the bar if you really want one I guess.
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OK, here goes.
If *I* were doing this, I'd prefer the 24/20 mix and 2 deco bottles. Even better would be 24/30 mix putting the end at about 32msw(approx 100fsw).
If this was a one day expedition or I was only doing 1 dive per day, I might be OK with the longer dive using only 50/50 as a deco gas if O2 availability was an issue. I'd want the He either way if possible. You want to be as clear headed as you can if you are going to be taking measurments and pictures (task loading) at depth.
Narcosis will make it harder to do this accurately and may reduce the amount of work you actualy get done each dive because you will be moving slower and you may have to repeat things you don't get right the first time... The cold wont help any with this either.
Because you are planning to do this in a repetitive, multi-day format, the extra gasses would (I think) be beneficial.
1: The shorter run-times means extra time not spent in the water that can be added to the planned SI for an additional safety buffer. You can add the combined time savings (I get almost 1/2 hour in v-planner) from both dives to the SI without making the days total run-time any longer.
EG:
|....dive1....|..............SI..............|....dive2....|
|..dive1..|..................SI..................|..dive2..|
2: A shorter run-time becomes even more important the colder the water is as well because you are spending less time just hanging there getting colder and colder while you decompress... You will end each day less fatigued due to having to burn less energy keeping warm underwater. This leads to being in better shape for the next days dives.
3: Since you will be doing decompression diving a ways offshore, having the extra 100% O2 on board would be a good thing in the event of any emergencies.
As to not wanting to carry the extra deco bottle. If they hang on your rig properly, they really are not in the way very much. If you are not comfortable diving with 2 sling bottles while doing other tasks as well, then get in the water and practice, practice, practice...
I'd plan to use 2 AL40s (50/50 & 100) on the dives you defined and they would(should) have more than enough gas for both dives based on my SAC rate (your mileage may vary). So, for a given day of 2 dives on your proposed trip, I'd require 2 sets of doubles and 2 sling bottles total. Well, throw in an argon bottle for the drysuit too.
If the first dive DID use up too much gas from the stages for some reason, I'd be reconsidering doing the second dive that day anyway untill the issue that caused it is fully resolved.
You might consider bringing extra stages & regs along (if available) in case of a reg or valve malfunction causing gas loss from a stage. Then you would not need to abort a second dive due to some simple to repair equipment malfunction or freeflow from the first dive.
