Not sure if I can really help much other than to confirm that these systems are complicated, as you've found already with that test of disconnecting the DLA. I similarly had convinced myself that the rear DLAs were weak, until swapping in an electrically good driver's DLA made them fully strong again. I'm assuming you've seen some of my posts on these issues.

On a used eBay DLA, they're probably pretty old by now. Two problems I have fixed on my driver's DLA are:
one of the internal motors was failing so I took the whole thing apart and replaced it.
The plastic external bracket that holds the cable sheath had started to crack, becoming flexible enough to make unlatching the door unreliable.
And the second DLA that was given to me by a fellow xoutpost member had that electrical problem.

So a used one can have problems. I'd consider that along with the pricing difference when deciding new or used.

Only other tip I can offer is that the Foxwell has an active test function to command things like the rear doors to lock / unlock (it does both of them together), so you can cycle through those tests, knowing that things like how hard you press the button are not affecting results.

When my driver's DLA had that electrical failure, I had the same symptoms as you do, except I'm pretty sure my center console button did not work. Whatever I wrote in my posts back in the day may correct this or anything else I'm saying here from memory.

Good luck and hope your key cylinder does not fail before you get this fixed.

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2001 X5 3.0i, 203k miles, AT, owned since 2014