Great analysis, I don't see any incentive to switch from SQ in your case. You might get better earn/burn ratios with BMI now (see British Midland forum for discussions) but with a more precarious financial situation and the likelihood that they will raise burn without increasing earn (like NZ), you have made a good choice. Also BMI doesn't credit SQ flights at the same rate as other STar Alliance in Business and First (only get half the points) so you lose there again.

UA doesn't offer much to you if you don't want to fly them.

Lastly, there is always the adage to join the program of the airline you fly the most . . . ^