Thanks Andrew,
I think I've got it figured out now - although you didn't explicitly cover one case: an example of which would be Elite and SE on the same PNR, using the Elite's eUpgrades, waitlisted for an upgrade prior to departure day.

I think that the result is: both will get E priority on the reservation waitlist and you will only clear if there are 2 R seats and you're next on the list.

If that interpretation is correct then I think it's easy to summarize the waitlist ranking as:

Prior to departure day:

- If two on the same PNR, both get the priority of the upgrade credit's owner on the reservation waitlist and you will only clear if there are 2 R seats and you're next on the list.

- If one person on PNR, priority is based on their own status no matter whose credits.

Airport waitlist:

- EVERYONE clears according to their own status, no matter whose credits.

Sorry if I'm incorrect and have further muddied the waters!