As a personal stylistic choice I always write “derailer”, which is the same way all Americans pronounce it.
> I am on a one-man campaign to replace the foreign spelling "dérailleur" with the English spelling and pronunciation "derailer."
English spelling often gives hints about where words were at the time of the Great Vowel Shift, when printing was just starting to standardize orthography. We got a mishmash of various places, and words that entered after can be spotted from words that entered before.
This is of course a much more modern word. But the French were the innovators in cycling and they got to set the word, and mostly the spelling (though we did lose the diacritical mark).
You can try to update the spelling, but that rarely works. Autocorrect will likely entrench weird English orthography further.
Oddly, if I type dérailler, my phone automatically adds the accent (because I have a French keyboard installed. That is a different French word, or rather, it's the infinitive "to detail". It's pronounced duh-rye-a.