Apple Watch 7 : Can iPhone 6s restore to watchOS 8?

Hi,

Have a customer that wants to keep their iPhone 6s running iOS 15.8.2.

Yes, I know it's old, and I've offered alternatives, but that's what they want.

They've seen a secondhand Apple Watch 7, originally compatible with iPhone 6s.

The watch has watchOS 10 on it, so obviously had been used with an iPhone XS or later.

Questions then.

If that watch is presented to the iPhone 6s, is there any way to get watchOS 8 on it? Would Watch setup on the 6s push watchOS 8 back to it, seeing that the hardware is a Series 7?

Are watchOS 8 firmware ipsw still being signed?

Effectively, if a once compatible Apple Watch is upgraded to newer watchOS versions, can it only ever be used with iPhones that are compatible with that version of watchOS?

TIA