I can't find any evidence for RACHAEL in the bible. In the Hebrew bible it's RAKHEL, same in the Greek Translation, and RAHEL in the Latin version. I don't know about older English translations or maybe French or maybe adaptions to literature. But I assume the E slipped in, just because it looked fanciful and would still be pronounced the same way. Or as you say, to make it look like MICHAEL or YAEL. But here of course the EL part means something: "God", or in the case of YAEL it's part of the vocabulary word.