Current struggles.
Most of my diving these days is not compatible with primary donate. I'm on a rebreather, I'm on my double hose, or I'm in OC sidemount.
Sidemount can be somewhat strategic to facilitate primary donate for most of the dive. You ALWAYS start the dive on the long hose. This is a very likely time for someone to need gas right as they get in the water. In a cave, as you near the turnaround point, you ALWAYS are on the long hose. For whatever reason that seems to be a fun time for regs to fail, don't know why, but it is what it is. You ALWAYS finish the dive on your long hose. Less important in a cave, but especially in OW, as you start getting close to the end of the dive, people are most likely to run out of gas. If you are on stages or deco bottles, then primary donate works as well. This strategy with your gas switching on sidemount allows you to comply with primary donate during the highest risk portions of the dive, and you hope that the other diver asks nicely for your gas. If they ask nicely, you have more than enough time to unclip, take a breath or two to check function and clear the housing since we are nice and consider divers, and off you go.
Double hose is one of those things that I just really enjoy when I dive single tanks in OW and it's personal preference. I am aware they used to teach buddy breathing on them, but for all intents and purposes it is not a configuration where primary donate is going to happen. That said, we will treat this exactly the same as we would a rebreather where you aren't going to pass the loop over for a myriad of reasons.
I am interested in how you dive the side mount rig, (never having done this myself).
You state that the start of the dive you are on the long hose, and that you are on the long hose at the turn point, and as you exit the cave, or toward the end of the dive, you are back on the long hose.
This appears similar to independent twins, where you enter on the long hose and do a switch very early, say at the bottom of the shot.
Enter on Long hose - switch (to short hose) at the bottom of the shot
Short hose - switch back at 150bar (to long hose)
Long hose - switch back at 100bar (to short hose) .. (turn point probably around 125 for open water dive)
short hose - switch back at 75 bar (to long hose). .. ( dive end towards 75 bar)
(The turn points are very dependent on what the plan is and how much redundancy will be needed)
I have been that diver that did the switch from the long hose at 100bar, only to find the short hose regulator compromised.
(Thats when I decided to only dive manifolded twins for serious dives.)
I find it slightly disappointing that so many people who use primary donate appear to have difficulty adapting to other techniques.
From a personal point of view, I must have been very lucky, those GUE/primary donate divers I have dived with have never had a problem adapting to diving with me.When I have been on CCR and primary donate is not an option.
Having dived independent twins, and with others diving independents, I have always expected to take the long hose, either from the shoulder bungy, or as PD, dependent on where it happened to be in my hour of need.
The one occasion where it all did go horribly wrong, I tapped my buddy on the shoulder and he handed over the long hose reg'. Initially, he though I was doing a drill, it was only when we transferred over to travel mix at the first stop, that he finally understood I had a proper issue. Goes to prove the best planned dives can go to ****.
As you say, things always go wrong at the worst point, one thing is never the issue it's when things cascade.
Interestingly, I was originally taught that a regulator being remove from your mouth would only occur if the diver panicked. Also, when donating, always to ensure the donated regulator was in the direct eye line to the OOA diver, obscuring your regulator. This reduced the risk of the diver taking your regulator from your mouth in panic.
Gareth