Is the B fare a refundable fare or one of those special non-refundable B fares?

If refundable, I'd buy the B fare and watch the upgrades like a hawk. You're still pretty far out from the trip, so there's decent odds that seats will open up for a standard WBC upgrade from B (60k R/T per person). If nothing opens up, hopefully you've also been watching regular coach fares on NW (and DL, CO, AF, KL, etc...) and you can just grab a cheap economy fare.

It's absolutely not worth 200k(!) per person for that upgrade IMO, but I'm not in your shoes, either.