Hi,
You are rite OOTB only one entity can be submitted, and Workflow or custom submit action with Dermis Bridge API can handle support mulitple entity.
I can handle via workflows but does not show up errors to the user in page so defeats the purpose.
Is there a particular reason you want to show errors to the end-user (form filler)? If all validations are in place, there should be nothing for the end-user to change to fix the submission issue. In fact, using AEM workflows is much better as administrators or backend personas can see the reason for the failure and retrigger the process if required.