My band from the first Apple Watch fits my Ultra. I’m happy they’ve lasted that long and look forward to new strap innovations whether they lose existing compatibility or not.

Double tap to lock. Always works for me.

You can do it yourself from the Watch app on the iPhone under Mobile Service then, I think, Add Plan (mine now says “Mobile plan O2”). If you’re setting up a watch from new you MAY be able to do it there as well, can’t remember though.

Just added my wife’s watch to her iPhone. Can confirm that it’s £7 per month.

Been using Watch OS since there was a Watch OS. Didn’t take long to get accustomed to the changes which, IMO, allow for cleaner watch faces. I also prefer the physical interactions with the crown and side button rather than swipes which can, under some circumstances, be difficult to do.