I'm not sure what Turing completeness has to do with such a problem. There's a theoretical non-Turing complete subset of HTML+JS that has all the problems you c

I'm not sure what Turing completeness has to do with such a problem. There's a theoretical non-Turing complete subset of HTML+JS that has all the problems you claim, meanwhile Markdown with a built-in lambda calculus interpreter is Turing complete yet much less problematic. I think you're overcomplicating things for yourself by bringing terms like 'Turing-complete content' into the discussion, surely it just suffices to explain that the more powerful the system, the more open to abuse it is?