I hope that the pro's responding to this thread have actually had experience retrofitting AC to a 1950's 1 1/2 story house. there are design features from 60+ years ago that can be a pain.
1. The tile in the bath looks original....very difficult to cut and not shake loose tiles....last job I did with those "tiles" also had a suspended 4" concrete slab underneath the bathroom. 1/2 day with a hammer drill to get the duct in
besides the "craftsman" that put that tile in 60 years ago is dead
2. I had to run exposed duct like that on an older house because the ceiling in the top floor wasn't really a ceiling.....just had furring strips attached to plaster.
3. I have had to place the registers in the exact same places as shown and make sure they had enough throw to get to the outside wall. because there was a beam preventing getting any farther to the outside wall. Personally, I would have used different grilles (commercial type) but floor registers have longer throws than ceiling registers.
4. Anytime I retrofit duct to an old house that has these design features-the homeowner knows EXACTLY where the duct HAS to go and who is going to be responsible to cover or repair the plaster. (especially plaster that cracks and falls off when you carefully try to cut it.)
So for the OP: call them back out to go over your concerns and let them take care of your issues and if what they say doesn't make sense post it here with pics.