My sympathies on the occasion of your diagnosis of a bad evaporator.
My suggestion -
The evaporator fittings connect to the expansion valve and are reasonably accessible. If it has not been done already, I would check the integrity of the evaporator through a vacuum or air pressure test before I put out for 16 hours of labor.
I would not want to second-guess your mechanic, but since the evaporator is the one place that can't be accessed in the system, it may be blamed because they can't find anything else.
Another positive test for evaporator leakage is to charge the system, get it running and "sniff" for freon coming out of the AC vents.
It is true that the original aluminum evaporators sort of dissolve over time, but I would make sure before I signed up for the repair.