My suggestions was to check for an electrical draw or to make sure all modules are going to sleep after 20-30 minutes. I am suspecting an electrical draw causing the issue(s).
There are other M3 owners with the same set up you have and have obsolutely no issues. We need to find the source of the problem. Our shop vehicle has a B2015 in it, and a 2 week storage period yields no problems and the vehicle starts just fine.