As Derech mentioned earlier the pasuk says that the kohanim were taken in place of the “peter rechem”s. This fits with Rashi who says that Eisav was created second but came out first.
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We see that Yacov had to buy the bechorah since eventhough he was created first the bechorah goes to whomever came out first.- peter rechem
However regarding the makas bechoros there is a chazal that explains why Hashem Himself had to kill the bechorim and not through a malach, because only Hashem can distinguish between a drop that produced a bechor and one that didn’t. This means that the bechorim that died in Makas bechoros were the fistborn to their father.
There are two reasons given for the taanis bechorim. I think that the Nafkah minah would be a C section. If we learn that they fast b/c of being spared in the Makah then all that matters is whether you are a first born to your father, however if it is for losing the avodah it would only apply to a peter rechem- one who opened his mother’s womb.