I think I'm saying that the "lemniscate-oval" description might be a bit of a red herring. Lemniscate, in English at least, has no connotations of "keyhole" that I can find. It is a mathematical term describing a figure that popularly we recognise as the infinity sign. It might be possible as Brian has posted that a translation from Italian into English created the "keyhole nebula", but that seems pretty iffy. Either John Herschel wrote it or he didn't. Selective quotes from Uranometrica aren't going to solve it, only a search through his original works.
Cheers -