The final insult: In Stage 4, steps 2&3, the "plastic cover" and "top part of the motherboard" don't fit into the new central cross. The rear strut on the old central cross (where the USB connector comes through) was straight across, whereas the new one is angled inward so that it interferes with both parts. (Perhaps there was a motherboard change between my original Mambo FPV and the replacement central cross?) I had to trim down the rear end of the cover so it would fit and also cut away part of the new strut to make room for the USB connector and part of the end of the motherboard (see somewhat sloppy job in the attached photo). The good news is that the central cross material cuts fairly easily with a razor blade.
Anyhow it's all back together now and flying again! This is a fairly difficult and tedious repair. I hope this thread will help the next guy who tries to replace his central cross...
Best Regards to All! -- JClarkW