A few thoughts:
1. Kudos for recognizing the reality of donating the primary. I have had 3 genuine OOA shares since I started diving in 1985. (About 1 share every 2000 dives - not a high incidence.) One of them was on a technical dive and was a very relaxed and well executed evolution as we both saw the failure and knew the gas share was coming in a few minutes. The other tow were recreational divers who were not even in my team. They were enough out of air and either too far from their buddy to reach them in time, or lacked confidence in their buddy being able to deliver. (There's a downside to being the experienced diver on the boat.) In both cases they just mugged me for the primary with none of the signals, etc that people OW divers are trained to expect. Unless the OOA diver is very close and the buddy very aware and attentive but the time the OOA diver gets to the donor, he or she won't have the time for the niceties of the air share protocol - they are just going to grab the primary right out of your mouth.
2. There have been a few other times when I have donated a long hose primary to another OW diver in cases where it made sense for them to use my gas and conserve their own. For example, a diver who was in the 500 psi range on the wrong side of a boat channel that needed to be crossed at depth before we could ascend and complete the dive. Yes, we could have surfaced and dealt with boats running us down on the surface, but it was very easy to just hand the dive my primary on a long hose and swim normally to the up line top preserve the diver's gas for an independent ascent. A 5', 6', or 7' long hose primary gives you a lot more options than a 36" or 40" octo.
3. You may take some heat on the selection of a SSI, but it works fine for your intended purpose at this point in your diving career - provided the corrugated inflator hose is long enough to enable you to have adequate unrestricted head movement. That in and of itself is a bit of a compromise however as a long corrugated inflator hose has its own set of downsides and you need to ensure that it is cleanly routed, doesn't drag the SS1 in the sand or mud, and doesn't become a little reef wrecking ball. The longer hose also increases the area where it might be located, making it harder to find in an emergency.
And of course, you need to check it by breathing fro it under water on each and every dive to ensure it works, as well as rinsing it thoroughly after every dive. The worst seconds stages I have ever worked on in terms of poor maintenance and condition have always been SS1, Air IIs, etc. Ignore their needs at your own peril.
The other downside of an SS1, Air 2, etc is that they don't breathe as well as a regular second stage and as you get into deeper diving that can become an issue.
You may find over time that an SS1 no longer meets your needs as well as a higher performing second stage on a short hose stored on a bungee necklace where it rests under you chin for quick and easy access after you've donated/been mugged for your long hose primary.
4. Miflex hoses are very torsionally rigid and don't always lay down cleanly. The casing will also eat the skin off the back of your beck in short order if you route it around the back of your neck and are not wearing a hood. They are not much kinder to hoods or dry suit neck seals either. I tried them and based on my experience with them I avoid them. They are a little lighter and more compact for travel but the benefits do not outweigh the negatives.
5. 40" hoses are too short, even for a recreational diver. Yes, you can route one under your arm and use swivel to make it work with a clean configuration, but it's still to short for use as a proper long hose primary. In addition, a 40 inch hose with a swivel or elbow routed under your arm and directly to your mouth is harder to locate and recover if it falls out or gets knocked out of your mouth. If instead you've routed a five foot, six foot or seven foot hose around your neck and it falls out of your mouth, it will usually still be sitting there on your upper chest and is very easy to locate and recover. Worst case, (if you lose it while inverted) the hose will still be across your chest making it easy to pull the second stage back up where you can quickly put it back in your mouth with no search, arm sweeping, etc, required.
6. How long is long enough?
Seven foot hoses are the standard for any technical divers, but it requires a BC with a waist belt into which you can tuck the extra hose (assuming you don't have a can light or proxy for a can light on your waist belt). The routing is straight down from the tank valve to your right side where it loops under the can light or is tucked into the waist belt and then routed across your chest and around your neck to come over right shoulder to your mouth.
A five foot hose works well for most recreational divers as it will route down from the tank valve, under you right arm, across your chest and around your neck coming over your right shoulder to the regulator in your mouth. It's nearly ideal as it routes cleanly even on a jacket style BC with no external waist belt, it routes cleanly around the body, it is easy to fully deploy my just tilting your head forward, and it is still long enough to allow easy swimming side by side, or stacked vertically above and below one another.
A six foot hose is ideal for divers with larger builds whose chest and shoulder dimensions make the extra 12 inches essential for adequate head movement all the way to the left.
7. A 90 degree or 120 degree elbow will keep the final bend much closer to you and present a cleaner configuration, and can add some usable length to the hose, but I don't recommend a swivel as they add more maintenance, more initial cost and more potential for failure of the swivel itself.
