I have put some comments on the PR.
I can confirm nothing is affected in the current meas_mosaic land when running with the two new branches on this ticket. I checked both the applyMosaicResultsCatalog & applyMosaicResults functions in updateExposure.py and they produce identical before & after corrected catalog and image results.
As for Jim Bosch's suggestion above, we agree this should be done, but can happen on its own ticket (i.e. with my confirmation above, it need not block this ticket). Can you please create that ticket?
Finally, the only reason I'm not yet marking this as reviewed is based on your comment in the commit message:
Fix bug in test, where it was using self.instFlux0Err instead of the passed
instFlux0Err (now calibrationErr). How this passed the tests before when the
photoCalib error was 0, I don't know, and it worries me. But its fixed now!
I'm not comfortable with this...if the tests shouldn't have passed before then they are faulty and no confidence can be placed on them passing (regardless of them now using the proper variables). Can you please try to get to the bottom of this?