Welcome to the forums!
Yes, I'm afraid this going to get you into problems. As a clarification, we need the Hyper-V role on that offhost proxy to do certain processing. But if the snapshot of that VM is from a different version, it is likely that you run into problems with compatibility issues or even get corruption on that snapshot. Especially in this case as the 2012 backup framework is completely different from the 2012 R2.
A few questions:
1. Are you planning to upgrade your 2012 hosts to 2012 R2 in a not so far future? If you are, I would propose that you use onhost proxies for now (given that you have some resources available).
2. Are you using a hardware VSS provider? Make sure that you have that one already installed on your hosts so that it can help you with the resources
Hope it helps