So, is there anything to worry about here, with setting the baro manually like we have? If the weather changes etc., altitude changes, will I have issues with the A/F ratio getting thrown out of whack? I'm sure the Baro reading has a strong influence on the fuel delivery calculation.
hell no, a 2 bar map with an 8 bit scale isn't nearly accurate enough for weather barometrics! the stock sensor is about 0.5kpa resolution. with your division you're more like 1kpa per step now.

think about it, what is your summer to winter kpa variation anyway? now divide it by two since you halved your kpa, i bet it's 0.02 or less.

tune for high air density, and when air density drops a bit from the weather, the extra juice wont hurt anything.

large altitude changes are an issue, but keep in mind even if you DID have baro update on startup, you'd have to pull over and restart the car every time you changed altitude, agin, that's why lots of factory turbo cars have a second sensor for baro pressure

boosted engines don't mind being a bit rich in the short term, all that extra combustion pressure and heat will burn off the excess no problem.

Also, should I expect to get the "high MAP sensor" code 33 when I go into boost? Obviously can't try that just yet.
i doubt it ...? but we could just disable that code. i don't think most speed density ECMS throw a default on map errors (if it did, it would probably be pretty nasty in this case)

if i have more time i'll trace all your map sensor code too and see if i can find anything like that, or other variables that need to be /2