Hi
@stepfen
Interesting, wide ranging thread, as often happens on SB
So, you want to get solo trained/certified. That's a good idea, for your safety, and if it is required to solo dive. As solo diving appears not to be allowed in your current two diving locations, the former seems more important for now. You never know though, perhaps you will expand where you dive and the certification will allow you to do so.
The training criteria from the various agencies, SDI, PADI, SSI, IANTD appears to be quite similar and I would imagine all are adequate. It is often debated whether they are equally recognized, I do not have the answer to that question. SDI's certification name, Solo Diver, has always seemed a little more direct than Self Reliant Diver, Independent Diving, or Self Sufficient Diver. I have the SDI cert and it has always been accepted by operators allowing solo diving with certification.
Choice of redundant gas is a personal one. Again, several acceptable alternatives. I chose a 19 cubic foot aluminum pony, more than adequate gas for any recreational dive for me. I sling it, it is always on, I have a SPG on it with a short hose. The regulator, valve, and SPG are right in front of me. It is about 2 pounds negative when full and a little under a pound negative when empty. I generally do not adjust my weighting, and don't notice it. If I carry an odd amount of weight, the extra goes on the right.
Solo generally does not include deco, and it would require recalculating your gas requirement, easy enough to do.