OK, so here's the plan for tomorrow. First, I will shut down the firewall in the server, and see if that changes anything. If it does, I will take firewall logs with the firewall on and off and debug from there. If as I suspect, doing so makes no change, I will reset the router back to factory default, and flash it back to stock. I will then run it as a simple router, with no VPN capability, to replicate the existing router.
If it won't do that, I shall contact the Asus support team, because my existing router does that acceptably and any router should do.
Assuming that I can make it behave as a simple router on stock firmware, I will then set up the VPN client (still on stock) and check that works. I know this will be of limited use under stock firmware capabilities, because it will stop external access to my servers (dns will still point at my static address, but my servers under stock capabilities will appear at the VPN termination), but I should be able to achieve internal (LAN) access, and that gives me useful debug info. (suppose I could change my DNS to point to the VPN termination, but that would be a horrible, and probably unreliable kludge which I shall avoid at all costs!)
Having got that far, and assuming it behaves itself, I will flash the router with the most recent version of Merlin, flush the config, and reconfigure it, again initially as a simple router with no VPN. This should work, or I will have exposed disparity between stock and Merlin fundamental operation.
Finally, I will set up the VPN and see if that works.
There are a lot of go/no-go points in the process which will help me understand what is working and exactly what isn't.
And to you guys out there who have been so supportive today (especially Colin), I thank you! You have come up with some good suggestions, and forced me to marshal my thoughts and describe the issue clearly, which is a major help when working on a problem like this where its all too easy to disappear down a rabbit hole. If any of the steps above could be changed for the better, let me know your thoughts.
I will update when I have got some concrete results