I live in a mobile home in North Florida and the 2 bedrooms at the west end are hot in the summer and cold in the winter. The ac/heat don't seem to get to those rooms. I don't even feel the air coming out of the vents. The line your ac guy gave you is a bunch of mularkey because the sewing machine doesn't give off that much heat. My 2 rooms get hot without anything being on.
What I did was to get a large standing fan, put it in the doorway facing the room I want to cool off and turn the fan on. The fan then brings the cool air from the other room into the hot (or cold) room. It works enough where the room gets somewhat comfortable. You do the same thing in the winter.