It appears that the permissions granted to the Home user has changed. It used to be that this account had the ability to setup DHCP reservations and port forwarding rules. Now, on current firmware, this is not possible and the Home user now appears to be a read-only account. Isn't that what the "readonly" account is for? Why would the behavior change between versions? We give customers this access to they can setup their own port forwarding if/when they need it, and not call our support department.
I'm sorry, I'm not following your reasoning. A customer can cause service interuption by changing the LAN address/mask improperly. And it's not clear to me how they can cause service interruption by setting a DHCP reservation or creating a port forwarding rule.