2. Are there any serious security issues in mixing OS and proprietary software?
I’ll quote Sorin Guiman again: “Security by obscurity is widely denigrated – it does not mean that if one doesn’t have access to the source code, that code cannot be cracked.”
One of the main advantages of open source software is that any security issue is transparently known and can be collectively & rapidly solved by the community developers, making the application at least as secure as the proprietary one. Thus the mix of the two cannot hurt more than mixing any types of software.