I've answered my own question. BPMN2 validation can be disabled by not using the BPMN2 Project Nature. This project nature causes automatic validation of all BPMN2 files; many of these rules are 'over-eager' when it comes to validation of Ad-Hoc processes.
When saving edits to a diagram, do not add the BPMN2 Project Nature, causing automatic validation of all BPMN2 files.
Alternatively, disable this using: Properties > BPMN2 > Uncheck "Check if project is configured for BPMN2 Project Nature".
This is strange as I find this ' over-eager' restrictions also, for basic things like: "ExclusiveGateway should have only one outgoing SequenceFlow" warning
From other forum I have read how to disable it:
After upgrading bpmn2 modeler plugin to 1.1.2 the problem occurred.. After looking into the source, I found out that there is a (new) option in the bpmn2 preferences, that gets involved on validation. To get rid of the error message, you have to check the 'Allow multiple incoming/outgoning on Activities' option under BPMN2 > Editor > Behavior.
Otherwise it is a problem in release 1.1.2 and is fixed for 1.1.4 and 1.2.0 as in here