Kerbal – Is it possible to transfer fuel with a probodobodyne OKTO in the middle

Should I even bother with trying a low energy transfer, utilizing Mun to get to Minmus?

No. Minimus has an eccentric orbit. Attempting to gain a gravity assist from the Mun that will provide the exact normal/antinormal force to end up in a good Minmus encounter at the exact right point of it's orbit will require some very exact calculations.

The most efficient way I know of is to match the orbital inclination before making the injection burn. From a parking orbit of approximately 100km this just happens to be as it rises over Kerbin.

Docking clamps have very little shear strength, so I suspect if we just changed the decoupler into two docking clamps, your cockpit would fall off during the ascent.

It's a bit more complicated to replace decouplers with docking ports; decouplers just fire off of your current stage, and stick to the stage you might want to return to. That doesn't work so well when you want the decoupler to detach from BOTH stages, leaving the docking port free for later use.

So what you want is:

• A docking port on top of your fuel tanks.
• A decoupler on it, upside-down, so your lander detaches from your extra fuel.
• A decoupler on top of the other one, right side up, under your engine, that will fire both decouplers off of your lander, and into space somewhere.
• A docking port somewhere on your lander. On the nose, if possible. Anywhere else will require you to dock on your side, which will make controlling it more difficult.

Pictures of this will be forthcoming shortly.

This setup will allow you to take it back to Kerbin (if you have enough fuel remaining), and leave it in orbit for future use.