*Mögest du von einem Schwertfisch gefickt werden.
Konjunktiv I, and “bei” would be appropriate for “by candlelight” or “at the bakery”, but not “by some agent”. “vom Bäcker” – you’re getting fucked by the baker, "beim Bäcker’ – you’re getting fucked at the bakery. Dative in both cases.
You know what also doesn’t make sense? Not boiling chicken in milk. I can guarantee you that’s not the milk of the chicken’s mother. The “don’t boil a young goat in the milk of its mother” thing at least has a proper interpretation in the sense of “there were some people who did that and God came and God said ‘yo that’s nasty, stop it’”. Something about not using sacrifice as an opportunity to practice transgression.
In the end I think scripture is just a tool for Jews to have something to argue about endlessly.