it appears with certainty that a macro rail is essential especially if focus stacking or no flash is being used
Definitely for focus stacking. The MP-E 65mm doesn't have separate focus and 'zoom' rings - there's one ring and rotating it changes both focal plane and magnification at the same time, so if you 'adjust focus' you're also changing magnification (making focus stacking useless) - so, you need the rail for that use.
But, you can use the MP-E 65mm without a macro rail if not using a flash, depending on the subject and your patience... This was shot tripod-mounted, with the lighting provided by a handheld Maglite LED flashlight.
EOS 5D Mark II, MP-E 65mm f/2.8 1-5x Macro @ 4x, 0.8 s, f/10, ISO 200
Did you say you had acquired a rail? someone mentioned the really right stuff rail but geez after blowing $1600 who wants to spend another 6 bills on the rail when those $50 china rails apparently work well according to to the reviews I've read..open to any thoughts?
I don't have a rail, but will probably order one today. The one I've been planning to get is the Velbon Super Mag Slider - well-reviewed and less expensive than the Novoflex/Kirk/RRS offerings. I shoot macro sometimes, but it's not my primary subject, and like Manfrotto tripods, the Velbon rail seems to be a good compromise between quality and value. The problem has been that B&H has been out of stock due to lack of supply from the manufacturer, every time I've looked over the past few months. But your question inspired me to check again, and they have them now. Thanks for the nudge!