Because they're Broadwell-e and not Skylake.
1. Yes, absolutely.
2. It depends. Was your Windows 7 licence a commercial one or an OEM? If the former you can probably install 10 directly and activate it with your 7 key (I think). If the latter you
may be able to deactivate the licence on your current computer and activate it on the new one.
3. Backup your documents and stuff (including appdata) and move them to the new locations, that'll transfer over most of your game saves that are not synced with Steamcloud.
Get a batch program installer such as
Ninite to save yourself a lot of time.
Sync your browsers to that all your plugins, passwords and cookies will be automatically ported over.
4. As long as you tell your new Steam install where your SteamLibrary folders are on the other hard drives there should be no problem. You'll have to reinstall the games but since the data is already there it should be quick.
E: apologies for the (almost-)double-post, I hit Submit instead of copy-pasting to my previous post.
E2: Don't know if anyone's seen the AMD RX 480 unveiling from last night, but if 2 8Gb 480s in CF fall somewhere between a 1070 and a 1080 for under 500€ I may just have a change of plans on my hands.