I read this rule multiple times too, figuring this would be our only point of failure. However, I *thought* it was meant to drive home the point that there would be no rapid rewards credits earned if you fly as a companion. Actually, now that you explain your method of reserving a 3rd empty seat, I can see this rule designed to prevent that?? The term travel as a member seems to infer Rapid Rewards membership and have nothing to do with a companion pass earner

If CP holders could check in online at all airports (it works at some and doesn't at others), then I'd try this out. I feel that even though the system would let me book with 1) my rapid reward 2) my companion and 3) my companion's companion, if we check in at the counter they might require passenger 2) to purchase a ticket.

5. An individual may not travel as both a Member and a Companion on the same flight. No Rapid Rewards flight credits will be awarded for flights taken by the Companion using the Companion Pass.

That's very creative using an empty seat like that, but I wouldn't be able to stand up to the pressure of explaining it 100 times to other passengers who want the seat. I think the stewardess would make me feel guilty as well.
On the plus side, by doing this you can very easily jump to the front of the line for involuntary denied boarding, get the free coupons etc, and still take the flight

Thanks for the warm welcome. I've lurked from time to time, when I signed up the approval to post wasn't automatic, and this was my first real ? anyway...