Just an update for anyone who runs across this.
The previous cell had issues with leaking, as I mentioned. The coupler could not be tightened enough to make a good seal under pressure, and tightening past a certain point would pop it off the threads of the SWG and of course the pressure would break the o-ring seal, dumping out water. The best I could ever get it was a slow drip. I used hose clamps, which helped, but did not fully resolve that. I did not try cutting the coupler.
I installed the replacement cell with the new coupler. This time, I could tighten it quite a bit more without that catastrophic failure happening. Fingers crossed it stays that way.
This is what I think, in my amateur opinion, what was going on with the original cell. The coupler threads were poorly designed (incorrect major/minor diameter or tolerance, something like that) or poorly molded. When the unweighted cell is horizontally mounted, it sits level with the plumbing and looks like it will couple just fine. As water fills the cell, it weighs the whole cell down. If the horizontal run is a bit long, because of the poorly designed/manufactured coupler, everything sags slightly, with the bottom of where the cell mates to the plumbing now looser than the top. The o-ring loses its seal and bam - water leaks. Because of the bad coupler design, you can’t get it tight enough without over tightening it. Adding tons of lube helps, adding lube to the threads helps, and of course clamping the coupler helps because the threads are closer to where they should be. Cutting the coupler removes a bit of material, and so clamping the cut coupler down fixes the diameter issue - so it is a much better fix.
With the new coupler, the sag still happens from the weighted cell - and it did leak on me at first - but now I can get it tight enough to squish the o-ring down.
It’s possible the mating conditions between the old coupler and old cell are actually the root cause, and not just the coupler.
Language in the pamphlet like “generational thread updates” sure sound like corporate-speak for “we knew about the issue but we don’t want to take responsibility for it.” They certainly knew from my dozen phone calls and back and forths with them, and it sounds like others did too, that their complaints/tech support department would have seen this as a trend. I know I only have my suspicions, and I know companies do this all the time, but this is the sort of experience that raises red flags when we audit companies - so I would suggest to go with another brand.