The OBB is basically half off the down-payment, which would be the same as a 10% discount on the total cost of the cruise, right? And could I do OBB for us and for another family traveling with us?
Since you mentioned December, just be aware that holidays are blacked-out for the 10% off OBB.
I totally get your quandary. Sailing out of CA would be so much easier for us than anywhere else. There just is so so little to choose from. I really hoped for something workable in the fall for us, too, b/c both DH and I are celebrating our 40th birthdays (10 days apart), but I just couldn't justify the cost for less than 48 hours. The only other options around the dates we were considering were Carnival cruises, which I don't have anything against per se (and certainly not against the newer ships), but I'm fairly certain I wouldn't be too happy with the shorter ones out of CA with my kids. The new NCL ship for Alaska looks to be sailing on the west coast in other months, so I will keep that one in mind as another west coast option, and Princess has a west coast itinerary that looks nice to me too if the dates would work out (one of their ships that sails in AK in the summer as well). I can't remember the dates off-hand, but it's not until late 2018 for the NCL ship, iirc. For the fall, we ended up deciding on Las Vegas in the end! Sounds odd, but everything I noted wanting to do for a proposed adults-only trip was actually pretty kid-friendly, go figure! We're not gamblers, just want to ride the gondolas in Venice, visit the giant M&M store, go up the Eiffel tower, see the Bellagio fountains, etc.
You may wish to price a December 7-night out of Galveston for 2017 to compare the cost/day vs the 2night Baja. We sailed out of Galveston over the winter break and thought it was a reasonable price for the trip, as it was comparable to other cruiselines' holiday rates out of FL. Galveston isn't a super popular port, I guess, and it does have fog issues, but we got lucky and had no problems on either end of the cruise.