James, sorry for chiming is so late! And thanks for your kind words. Kimmers, nice thoughts and ideas as usual -- despite your troubles (so sorry).
I can only answer your questions as an active member in this community during the past four years and as a single-level ADR patient -- the outcome of what may be your future surgery is dependent on SO many factors. I think you know this already.
Regarding hybrids, the outcomes are (as mentioned before) somewhat variable. IMHO, inconclusive for myriad reasons. Some have done extremely well, others not so well -- this measured in their words describing their pain relief over a one to three year period. Perhaps some of the explanation for this is the (possible) higher risk that multilevel patients have in terms of the evolved pathology of their degenerative spine disease(s). And arthritic conditions that remain elusive or undiagnosed may be a missing piece of this terrible puzzle.
Regarding movement at L5-S1, it's most definitely there. Though comparatively small, it is important. Is there less movement? Maybe. More stress and compression? Probably. Whatever the case, it's difficult for us to "decode" what the studies are saying on this important & complicated topic.
James, have you carefully reviewed the "risks and complications" post in the Arthroplasty Central forum? This may help you articulate the "tough" questions to your doctors. If you are contraindicated for ADR, but not fusion, then some type of fusion procedure may be a good choice if you do require surgery. Perhaps you would benefit by receiving some other medical opinions?