My masseter shouldn't reacquire its full size unless I expose my muscles to the same load?

The effects of Botox are temporary. Long term changes could lead to improvement, but it is unlikely that you will notice permanent differences after only one treatment. I encourage you to discuss your concerns with your injector.

Yes, if you no longer chew or clench. However that is not realistic. Even not eating solid foods, some patients still clench or chew unknowingly during the day and especially during sleep.
With regular injections, you may find the dosing will diminish and in some patients, the time between treatments will increase significantly while the dose decreases.