Gertau: I thought that any stroke game would contribute to my average even a multi-player game.
You think right if it's ranked play. The dick is elsewhere, assuming only A/S and matches.
For the fun of it, playing the rocks in MPs may be a cost reducing solution. Alas, it won't advance you, given the difficulties.
OTOH, a good dedication and a lot more bucks are necessary to tier up to TL. I'm on 30 Euros per year, an eternal Legend with a mere -.5 in the average in six months.
Furthermore, me thinks that MP games should only be played for fun, nothing else. Not for average, not for CDP, not for XPs.