I don’t play violin or viola, but DO play ukulele .... which lies across your chest and puts pressure on the actual boob area. I found it took a while longer than I thought to ‘accept’ this pressure on the wound area even tho I had a lumpectomy, not mastectomy and was sleeping ok on my tummy already! Weird!
With a mastectomy, I would think the main problem area would be in lifting your arm up into the ‘shoulder hold’ posi5ion, depending on how many nodes are removed and whether there is an incision under your arm as well as the boob area.
Everyone is unique in their wound recovery - I hope your recovery goes well and maybe just make haste slowly so that you get your full range of movement back so you can get back playing again.
Good luck on your journey - hoping for the best outcome for you.