With 2 servers and the file share witness in one of those data centers, you will have an outage if the primary goes down.
The only way to achieve high availibility is to have a 3rd data center with the File Share Witness located there.
Otherwise, you will be doing a data center switchover
Here is the doc for that:
Its preferred to use the same internal URLs, yes.
You dont need to separate networks for MAPI and REPL. One is preferred:
If you are not using a load balancer, then you would need to remove the primary from the round robin DNS that points to the client access endpoints in the event the primary is down
I recommend at least 3 servers, not 2. Two in the primary, one in the backup data center