[Ethereum] Should I consider building a mining rig with PoS being so close

1. Perfs are almost identical but your ROI will be much better with Ubuntu as it's a free OS. I assume you won't run Windows without a license. Personally I prefer Linux as it's much more easy to setup and configure and works without maintenance. It's also easy to script. Use Ubuntu 14.04.
2. Yes you can mix non crossfire and crossfire cards. Just take care to have all cards ships of the same founder. Don't mix AMD and nvidia cards, you would have problems with drivers.
3. Booting on one of the mining cards it ok. If you use Linux, it doesn't impact performances if you don't run 3D things while you mine. I don't know about Windows.
4. You can run VM as it mostly use the CPU and mining is more on the GPU but be aware that anything will heat instead of just GPUs, so you'd better have a very good cooling system.

If I understand you correctly, you're asking how a node, in PoS, can trust another node it is connecting to for the purpose of downloading the blockchain, as it with PoS quite easy to "fake" the entire blockchain from the beginning of time (generating blocks requires little work).

PoW, on the other hand, is impossible to fake unless you buy a very very very large amount of computing power and generate valid blocks (maybe leaving out a transaction or two to give yourself some extra Eth).

To solve this issue, see this blog post about weak subjectivity. Essentially, when you sync your PoS node for the first time, you are going to have to go to a source you trust, whether that be the internet, a block explorer, Reddit, or a node you trust.