"I have no idea what I'm doing"

Also, the editor needs to produce output using the new templates, while still understanding the old ones alongside the new. That's the part where I got stuck.

I think it works now. New translation requests will use {{t-needed}}, the script will recognise lines using {{ttbc}}, and will correctly remove correct usages of both {{trreq}} and {{t-needed}} (and some incorrect ones). I have kept {{trreq}} as the magic string the user has to type as the translation, not to break users' habits. I am still not quite sure what exactly my changes did. Please test.

I must say, that script is a giant pile of hacks, and needs to be rewritten from scratch. It simply cannot be maintained as it is for much longer.

I agree. My strategy is usually to rewrite parts over time, rather than to make everything new out of nothing. But if you can make it easier to maintain, that would be very nice.

Thank you for taking the time to do this anyway.