Patrick mentioned a similar theme when he talked about concentrating on the image
rather than the "controls". I am sure that when/ if you add a 21mm it will become second nature on how close to stand (vs. the 35mm). I think that one body/ one lens thing is only in order to get to know that first lens (and the body)... after that it comes quite natural
I was reading that the M9 has bright-line frames for 35 & 135 mm, 28 & 90 mm, 50 & 75 mm... I guess you will end up using 28/90 rather than a 21mm external finder. To be honest, with my 15mm, I just shoot the using the entire window and ignore the bright-line frames.
No hurry on that temple series... spend some time with it.