Can't make rhyme or reason of the MTM pricing but here we are:
1. SFO-JFK-LGW for August. Required 1 cert for SFO-JFK + 2 certs JFK-LGW = 3 certs total. Fare difference around $1700. If I'd just booked JFK-LGW, fare difference on that sector would have been around $900.
2. SEA-JFK for December. Required 3 certs (only redeye with a lie-flat, and completely inflexible on when I need to fly, so I bit the bullet). Fare difference $550.

Not thrilled about #2 but at the end of the day, 6 certs - from the DL status match -- burned on SFO-JFK, JFK-LGW, and SEA-JFK isn't a bad haul. This, plus some other paid flying and CC spend, might just be enough to help me get to Mosaic 3!