You're not just going ahead and changing this! That's not the way we do things. If you do we will have a problem.
However, we can discuss it rationally if you want. My issues with this are:
a) Late changes. Introducing what is essentially a new feature, which would need to be tested and confirmed by QA. You will never get QA guys to do this, they are swamped. You'd have to write an automated test for this setup at the very least, but QA would still need to have time to verify. This would have to be accepted by QA first or it would be marked as tech preview.
b) The setup doesn't make sense. Most people use NGINX or Apache for loadbalancer, not Undertow.
c) The setup won't actually work. H2 kinda works if you point to a shared database, but not properly. H2 lazy writes changes to file, so it wouldn't work for concurrent requests. You'd have to setup an embedded H2 server with the TCP connection.
d) Domain mode is not just for clustering. It's to manage groups of servers. A perfectly valid domain mode setup could be one domain controller, one group with one EAP instance, one group with a RH SSO instance and another group with two RH SSO instances in a cluster.