A question: in a poem, is it alright to liken the first-person (autobiographical, in this case) narrator to one figure in one stanza, and a second figure in another? Does that count as mixed metaphor? In my poem, I have: / exiting and again entering sweet Eden / - but, later on: / Must I be the wakeful in Gethsemane? / Shall I change the "Eden" Eve reference line to another Christ reference, or is this acceptable?
No, this doesn't count as a mixed metaphor. A mixed metaphor combines two unrelated metaphors. What you're doing here is not even a comparison. You're simply saying the narrator is in two different gardens.
I actually like the contrast between the paradise of the Garden of Eden versus the hell of the Garden of Gethsemane. A paradox such as that is what poetry is all about. Even if it were a mixed metaphor, it FEELS right.