No, you can't have a Coordinator on one PAN discover nodes from a different PAN.
No, there really is no way to reduce that time. Not with a network of that size.
While the 868 LP has a sleep option, it is not enabled by default. You must enable it for that function to exist.
No, Digi no longer offers an 868 PRO module.
Yes you can tell your remote module to send a status change if Change Detect is enabled. Your Coordinator would also need to be in API mode for that to work.