Even stranger. Once the 2 routers have linked up, I can delete the one IPv6 from the MT router and the Netgear continues to work just fine using what should be an invalid WAN IP at that point. Once I reboot the Netgear router though, it loses internet again.
I need a stiff drink!
That's because SLAAC works in some ways like DHCP does... Basically, if you delete a host's lease from a DHCP server, that host isn't going to suddenly stop working - it's going to continue using the information until the lease expires. There's a lifetime associated with the RA messages that SLAAC uses. Any host performing SLAAC will not bother checking the interface again until the lifetime has expired. If it hears a new RA from the same router with different info, then I'm pretty sure that the host will update its settings accordingly at that moment as well.
After a bit of reflection, I would warn against using any "private" IPv6 addresses on the WAN links if Netgears use RA on their WAN interfaces - because those routers will send/receive packets towards the Internet using their WAN interfaces (almost certainly) and thus if the user clicks the "update firmware automatically" button, and the Netgear tries to use IPv6 to do so, it will fail as the source address would be fdxx::/8 private space... (devices BEHIND the Netgear would have no such issue of course)
EDIT: As for the PD working for Mikrotik issue - notice that they have a checkbox in the IPv6 DHCP-PD Client configuration: "add default route"
This is a workaround added by Mikrotik, as I don't think there's any option in DHCPv6-PD that actually passes a default GW to the client. It's just expected that the client will also be listening for RA messages. So in a nutshell, the Mikrotik client just makes the assumption that whatever host (link-local address, BTW) sent the DHCP response, that host must be the default GW. Netgear apparently has no such behavior (unless it's there and you haven't noticed it yet).