Yes, retail copies of Windows 7 could be legally moved from an old system to a new system as long as the old system is no longer being used.
Yes, Windows 10 allows moving from one system to another (even OEM) if the OS is tied to a Microsoft account. I haven't yet checked if this is still the case when you use a local account instead but would assume the same 'retail' rules apply. Again, this is assuming that the old system is no longer functional.
Yes, Microsoft is being an asshat about discontinuing older OS support for newer CPU's.
Yes, on the Intel side Skylake is still okay for Windows 7/8. Kaby Lake is a no go for 'supported' operation.
Edit - If enough corporations fight M$ on this M$ may back down. Legally, this is new...