no bendy risk, but an embolism risk if you don't do it properly.
Of course there is a DCS risk from a CESA. You are rapidly decompressing with no safety stop, and far faster than the recommended ascent rate. It's probably minimal if you are practicing a skill and just descending and ascending, but if you do it late in a dive near your NDLs, why wouldn't that increase your decompression stress?
IIRC NAUI still requires it as a skill in OWT.
It's a required skill, but it's certainly a last choice for an OOG emergency (first choice would be another gas supply), so I wouldn't agree with the proposition that a diver doesn't need redundant gas because they can always CESA.
Here's my problem with pony bottles. If diving a pony bottle, it is not going to be my donate bottle because I donate out of my mouth. This means that my long hose is still on my backgas bottle and then the pony is the secondary on a suicide strap. Very annoying to deal with due to hose routing and you can't just pass it over to someone. Alternatively you can dive a normal reg configuration on your backgas and have the pony as something you can hand over, but now you need to donate normally, then get the OOA guy to switch bottles which is easier said than done. Means you now have to switch regulators which can get interesting depending on the situation. Not to say it can't be done, but probably more of a PITA than necessary.
I dunno, I think that you may be overthinking this. Leaving aside the specifics of the OP's planned dive, lets just consider a relatively new recreational diver with a single tank, who is diving a bit deeper than their comfort zone. The easiest single thing to add to their rig to address gas redundancy is a pony bottle, slung, with a second stage bungeed against the tank. That way, whether they are diving long hose or traditional configuration, if something happens to their primary gas supply (i.e. free flow, first stage failure, etc..), they can just grab the pony regulator. Tank open or closed is a controversy, but either way, it's nice to know that sitting at your side is a completely redundant gas supply. And this is for the individual diver, not for sharing. Slinging a pony doesn't mean that you can't donate your primary or have someone breath off of your octopus if you aren't running a long hose.
If diving a mounted pony, then why not just dive doubles? Doesn't have to be manifolded doubles, can very easily be a set of independent doubles with cam band doubles bands so you can put any cylinder in there that you want and while a pair of mirrored valves is ideal, it isn't necessary.
More cost, more weight, non-trivial skill set (manipulating back gas tank valves), especially in an emergency. Nothing simpler than a pony hanging under your left arm, with the valve and reg right were you can see and check them. It's such a simple, cheap, reliable solution. Plenty of dive ops will require them for single tank divers on deeper dives (that happens on some of our local boats, and was strongly suggested at Truk this year). Doesn't require assembling mismatched doubles with those slippery cam bands (have had them fall apart underwater), doesn't require messing with a bracket or finding a reg behind your back.
Sidemount is ideal if you want fully independent cylinders though I don't like boat/shore diving with sidemount.
Me neither.
