It's deadlocking because there's some statement earlier in the transaction holding other locks. You need to identify what those other statements are before you can solve this deadlock problem.
Don't...

For when the procedure was created, look at the create date in sys.procedures. SQL doesn't track the last execution date except as tied to the plan (which is not necessarily...

Yes, it will take a while if there's stuff running. It doesn't need single user, but it can't complete until all requests running at the point it started have finished.

No, it means that 20 devices max can connect to that single SQL Server

The first allows you to use SET TRANSACTION ISOLATION LEVEL SNAPSHOT in your queries. SNAPSHOT cannot be made a default isolation level
The second makes   changes the default isolation level...

Understand what about that?

Correct, that table won't be there, it's not expected to be there.

Yes, no, no

They just must not take their own transaction log backup.

Edit:  And you can carry on just restoring the log backups on the sunday, the...

A CTE must be followed immediately by a query. Yours is not, it's followed by an IF statement.
The IF needs to go outside of the CTE.