I haven't been trained for deep, coldwater dives, so I don't know how to handle the first point. To me, that's a big-boy dive. I am guessing that double tanks would make more sense to me than a pony. Besides, with a drysuit, the weight of steel doubles is going to be useful.
Not all deep, coldwater dives are in drysuits.
There are rec diving protocols for what to do when buddies lose each other, etc., like look around for a minute and then surface.
The problem with this is that it is a wide window. The amount of time required to identify the absence of a buddy, plus the one minute, plus the time to ascend, allows a lot to go wrong. Remember that it's not just OOA due to failure to monitor the gas supply that we're concerned with, it's things like 1st stage failure.
But reading some of these comments, it's as though they are saying that single-tank minimalist diving is inadequate. That's crazy to me. Single tank diving is what attracts millions to this sport. They don't all need ponies. They MAY need better buddy training.
I make lots of dives without redundant air, some of them because I have a buddy who I trust. It really depends on the diving you do. Some of the best, most enjoyable diving in the world is in an environment where redundant air is not necessary because buddy diving works out so well. There are other dives where I prefer a pony because of a combination of depth, visibility, and who I'm diving with. It depends.
I disagree with the last part of that sentence ["intentional planned use of the gas in the pony cylinder at some point in the dive."]. I have not been trained in using a pony or read up on it, and my thoughts are just off the cuff, but it seems to me that a pony should not be considered as usable gas.
Nomenclature:
Pony - small cylinder used in addition to primary gas supply regardless of purpose
Bailout - gas reserved for emergency purposes
Stage - cylinder other than the primary one that is intended to be used for a particular stage of the dive
The key thing here is that a pony is just a gas cylinder and its intended use is one you can define during gas planning. If you want to use a pony exclusively for bailout, great. Many liveaboards and charters only allow that type of use. If you want to use it as a stage, there's nothing wrong with that, but you have to be mindful of your dive planning. You don't need to be a technical diver to use a stage as long as the dive is staying within recreational limits and you're not switching gases during the dive (e.g. from air to nitrox). There isn't much difference planning wise between diving an AL80 with an AL40 pony to extend the dive time and diving a single HP120. Again the thing here is that you have to plan and you have to know what you're doing. If your diving is going in this direction, run your experiments in a benign environment and seek mentorship if you need it.
So how does one know when to employ which? Switching back and forth between using doubles on this dive and a pony on that dive because (hypothetically) the dives I like to do straddle some threshold doesn't sit well with me. I like consistency. If I'm on the threshold of needing more gas or redundancy, my thinking is to go the conservative route on all those dives and take the doubles, even if it's overkill for some of the dives.
If you can dive doubles on all your dives and you want to, great.
Echoing your earlier comments, I will say that single-tank minimalist diving is fun and is usually why people decide to start diving. Most of my dives are shallow, and I don't carry redundant air.
When I feel that I need redundant air for a dive, I will usually use doubles now unless there's a reason I can't. Most often my reason for leaving doubles at home is air travel.
