While I cannot speak for the intent of the mechanic, or the costs incurred (every vehicle is different) I can answer some points in your questions.
Replacing the schrader valves will require a system vacuum and recharge as if the schrader valves leak, the freon has escaped and has likely normalized with atmospheric air. This means that the system has become contaminated and will need to be flushed, vacuum drawn, and refilled. It is a very time consuming process, especially if other leaks are present, which can be found once a vacuum is drawn on the system.
As for the motor mounts, It depends on the mileage of your vehicle. Some vehicles are prone to premature motor mount failure. If your vehicle leaks, or has not been maintained regularly, this can also contribute to mount failure. Replacing all of the motor mounts should not normally be necessary, however I've run into cases where literally every motor mount had to be replaced, and that in itself is not an easy feat. The motor must be supported, and some mounts may be in difficult portions of the engine bay and may not be easy to get to.
You should always go get a second opinion. Do not take one mechanic's word for it, especially if you feel or suspect he's giving you a run for your money.