I 100% agree with your assessment of トリガー法, that it's probably translated on the fly in many cases (though this changes the more it gets referenced of course), that it's used in a very limited (and recent) context. But:

1) The fact that it might be perfectly obvious to any Japanese news editor or US-based journalist that "trigger law" would most naturally be translated to トリガー法 rather than トリガーロー or say 引き金法律, it won't at all obvious to plenty of JSL learners. 
2) Even if the context is recent and limited, it's a word anybody reading English-language news will be aware of. It's not a single law it's referring to either.
3) again, it seems likely to me this is the kind of word users might search for. I think it makes sense to to err on the side of inclusion here.

That said I guess it's possible "trigger laws" are largely irrelevant now that Roe v Wade has actually been overturned. (OTOH they might become relevant again as one judge of the Supreme Court has flagged more cases might end up overturned)

I think we need more than that to justify its inclusion.
"Trigger law" is used almost exclusively in the context of US abortion laws. Pre-2022 online results for "トリガー法" are virtually non-existent. It's a direct translation, probably done on the fly in most cases. I don't think it's appropriate for a JE dictionary.

OTOH people use jisho etc. for reverse look-ups all the time, and this is certainly the kind of thing an American wanting to explain something like this might look up. Or an E-J translator. It won't be 100% obvious to everyone that "trigger law" would be translated as "トリガー法" rather than "トリガーロー".

Yes, a transcription of an English term.

I don't think this is needed. It's only in the news because of Roe v. Wade. We have トリガー/trigger as an entry. I think that's sufficient.