Graphite Watch and Graphite Milanese are PVD coated to prevent color from rubbing off and they can damage each other.

The Space Black is DLC coated and so was the Space Black Milanese loop and those could also scratch each other. This has been known since the Space Black Watch and matching Milanese were released back on the S3.

Raw SS micro scratches can be polished PVD/DLC cannot. Sapphire can scratch Sapphire. Diamond can scratch Diamond. Materials of the same hardness can scratch each other.

Nah no longer supported outside of security updates going forward.

WatchOS 10 is only compatible with iOS17 and up. Since your SE is still on 9 it’s still compatible with iOS 16. WatchOS 10 removed the legacy support for past iOS versions.

Zero issues with my Series 4, I made a post about it.

Yep usually after an update and the OS indexes everything is back to business as usual. This happens basically all the time but they allow people to panic post here for some reason.