Non-surgical Alternative to Botox for Reducing Masseter Muscles?

 Are you sure that the problem is a prominent masseter muscle??
If yes, then the other suggestions make sense.  Sometimes, the buccal fat pad can make the cheeks look fuller.  Reduction of the fat pads is an easier procedure vs masseter muscle reduction.

You are asking an impossible question. The only solution I can think is this;
1. Stick to soft foods that require very little to no chewing. Even soft chewy foods like bagels or ganola bars can be bad.
2. Reduce the clenching of your jaw. To accomplish that during the day, buy a wrist watch that has an hourly beeping feature. It will remind you all day long to relax your jaw. For the night time clenching, 2 minutes before falling asleep do some auto suggestion, i.e., tell yourself, I will not clench in my dreams tonight. If this technique fails then see a psychotherapist to help you achieve this.
Goodluck. If all of the above does not work, don't be resistant to Botox. It is very safe and works great.
Regards