been running my prophet single ring up front for a while as I've been to lazy / underfunded to sort it out. the little plastic thing and the nut on the bb underside has been lost and leisure lakes don't get any in until October. is it possible to run a derailleur which doesn't require under bb cable routing on a prophet?

The plastic thing aint prophet specific, anyone will do. Likewise just a nut that fits. Not hard to come by both.

No, the cable needs to go that way, as that's where the cable stops are.