Hi @peteratomic thanks for taking the time to provide more information thats very helpful. I understand if you don’t respond after this but I will provide some information from my understanding.
utterly simple. an iframe wrapped inside a paragraph. consistently erased by the theme.?
I see looks like you want to add a class to the block, most probably for customization?
It would not be possible to have an iframe code inside paragraph tags, the editor Gutenberg automatically strips this. The best way is to just add the iframe code to an HTML block, if you want to customize the area around the iframe code I would suggest moving the HTML block into a Group Block.
The Group block comes with several design tools which might help with customisation.