Changing the provision server in the firmware will not be of any help.
The important part is that the provisioning cert must include the same domain as the option 15 setting.
So what you will need to do is get a multiple domain provisioning certificate, which should include all your domains within the request. (ie horse.local, bamits.local, domain1, etc)
Yes by entering the MEBx and entering the FQDN in the Network Name Setting section will work. This is due to this data will override any information passed to the MEBx from the DHCP Option 15.
This will allow the provisioning process to commence, however during the process the FQDN will be altered to that specified within the provisioning profile