::Well I roughly agree with you there but to educate you have to explain. I am in rough agreement with you and I don't hthink "datums" is wrong: it you say it's right I accept that, but to an intellegent but un-knowledgeable reader it is wrong; assume maximum intelligence, minimum knowledge. Though I understands your gist for a footnote, I wonder if that is the best solution: seems to me with minor rewordinf you could just keep it as "datum" and avoid the problem, unless (which I can't see to be the case) it's deliberately introduced to teach the plural. 

 ::As I say I have no problem with the plural as such English can do what it likes and anyway (per my analogy with media/mediums) it forms what Fowler called a useful distinction; I jsut wonder whether in its specific habitat here it's confusing or distracting. 

 ::Best wishes