Involved in potential friction in department over a postgraduate seminar I co-organised - should I worry?
I don't see anything that you should be worried about, as you've said in your question, your supervisor is the DPD and is the one who approved everything you did.

This head of research group probably didn't know that your activities were approved by the DPD and just thought this was a bunch of students adding to his workload. My expectation is that the DPD speaks with this head of group and they sort things out between them. Your personal supervisor will have your back on this one and has more clout than the group head.

I share the concerns about health and safety though. That sounds like it should be done by a health and safety professional, which your institute will have, rather than student lead.