Out of interest, can you add the listener after you've created the AG?

When you try to connect to the listener, what happens (is there an error message, etc.)?

Port 1433 should work ok for the listener; have you tried it?

I'm not sure that's strictly an in-place downgrade- isn't it just an uninstall and reinstall in disguise :)?

You don't say what you're upgrading from and to, but don't do an in-place upgrade in Production without at least testing it elsewhere first. (In fact I'd say don't do...

Unless I've misunderstood, I think the term you're looking for is a multi-subnet failover cluster?

https://docs.microsoft.com/en-us/sql/sql-server/failover-clusters/windows/sql-server-multi-subnet-clustering-sql-server?view=sql-server-2017

It would be prudent to do some testing first because of the new cardinality estimator that came in in 2014.

My inlaws really, really don't get it, and my father persists in thinking that I work in InfoSec, even though I never have, so it's quite hard-going round my way 

I  haven't tried it, but I assume it's because backups taken on secondary replicas have to be copy-only.

Why do you need to make primary the preferred backup location...

I don't want to switch to another one, so maybe I'd apply for something totally different. I don't know, National Park Ranger. (Obviously that would pay a lot less.)

Try switching to the master database and then running the restore code again. Or you could do alter database NAME set restricted_user with rollback immediate first, then try again.

I have many questions!

- Are you restoring the database yourself each day, or is somebody doing it for you? 
- Has the restore definitely finished when you go...