I don't really see anything verifying one way or the other, whether Fallout 4 will make use of DX12, but I'm wondering whether I ought to preload the game in Windows 7 or Windows 10. Most of my stuff is in Windows 7, so it's more convenient there, but additional performance is hard to beat.

Very doubtful, also very doubtful you will see any game incorporating DX12 for some time.

Yeah, it's my understanding that developers who aren't creating exclusively for the Xbox don't have a lot of incentive to use DirectX 12 because the PS4 doesn't have anything similar. The PS4 has sold a huge number of units, so most third party developers are going to go multi-platform and stick to technology common to both boxes.

Also it does not matter which OS you install under. Fallout 4 is just an application and will run fine on either. After you upgrade, it will still run fine - unless you do a clean install over it.

It is my understanding that Fallout 4 uses Dx11, but don't quote me on that.