Not sure how much help this will be, but I'll contribute anyways in case it may help someone sometime.
97 Pontiac Grand Am
Century Ascend, Cosco Touriva (FF), Evenflo Express
All in harnessed mode, installed with seatbelts, no tethers (don't think there were any.)
Very, VERY tight install -- if you're going to try this (or similar) expect scraped up hands. I believe it touched the door on at least one side; I didn't know any better at the time. Someone with more experience may have been able to achieve a better install; I thought being able to close the doors was victory enough. We rode this way for about a year.
2002 GMC Safari
Have had many combinations of car seats in here -- just listing a few...
All installed with seatbelts, l/s-l-l/s, listed driver-middle-passenger.
2nd Row
Cosco Booster*/Evenflo Express/Snugride w/o base (may have fit with, never tried it)
Cosco Booster*/Snugride w/base/Evenflo Express
Cosco HBB/Snugride w/base/Nautilus
Cosco HBB/Snugride w/base/Graco Airbooster
MR65/Snugride w/base/Cosco HBB**
3rd Row
Apex/Scenera FF/Apex
Apex/Scenera RF/Apex
Cosco Highrise/Evenflo Express/Cosco Highrise
Cosco Highrise/Scenera FF/Highrise
Highrise/Scenera/Cosco Summit
To be honest... I find it hard to believe that any combination of seats couldn't be somehow worked out in this van. We have to do a bit of crawling under/around to get kids INTO their seats, and it helps to have kids who can buckle (or mostly buckle) their harnesses, but you do what you have to do. (Pre-reharnessing, my rule was that whomever wanted to sit in the "back corner" had to be able to buckle themselves.) Worst seat to buckle by far is the Scenera in the Apex config, but I think that has more to do with it being a RFing Scenera and less to do with the configuration.
*Not sure model name... high back booster that has no harness. Cosco or Dorel something. (Voyager?) We always called it "brown seat," ha ha.
**Not 100% sure this config is an acceptable install, due to the play between the SR and the MR, but I'm guessing if the psg seat can be moved forward, you could put the Snugride outboard and the HBB in the middle.
None of these were tech-checked, btw... I'm very much not a tech, but my car seats do NOT move once I've installed them.