I can see the confusion.
Take this example.
K5 in the pic is a BUILDING (not orchard)
MMG unit doesn't exist.
Does K5 become your destination hex due to the Closest in MF? as you are ignoring the rout restrictions.
You are saying closest first and then on your way, you have restrictions VS the opposite of choosing LEGAL hexes that you can enter in determining Rout locations.
As most suggest in the 5MF distance Locations
If you ignore the legality and just focus on the "pure' MF needed, then you'd have to select the building hexes that are 3 hexes away, or for that matter Enemy location hexes.
Do you see how it breaks down if you go by PURE MF distance without first implementing the restrictions?
ASLRB
"Assuming it can abide by the previous requirements, a routing unit must move to the nearest (in MF calculated at the start of its RtPh) building or woods..."
My summary so far:
"I find locating the closest woods/building, determining if it is ignorable or not, and then determining whether one can legally reach it or not (and then rinse-repeat if needed) is the best way to stay in the rules."
The K5 building would be closest but it can not be legally reached so I believe my summary holds true in practical terms. However, looking over the Comprehensive Examples, it seems as the intent is that if you can't reach anything at 1 MF, then you check the 2 MF locations, then the 3 MF locations, etc up to 6 MF. If you still can't reach a woods/building, *then* you can go anywhere. I extrapolate this from the phrase: "Able to ignore all rout destinations within 6 MF, the broken unit is free to rout anywhere it can legally reach on the board [10.51]."
Either way, I believe one only cares about the "base" MF for calculating closest, not the MF that could possibly be spent. Why? Well I'm not seeing any instruction to do so and if you're the DEFENDER, that theoretical MF could change whereas the "true" MF never does. The only conditions I see in the RB are that the true route is through a minefield/FFE or otherwise not traversable (can't jump off a Cliff into the building, etc).
Probably worth a Perry Sez, I think this image and question would sum up the question:
Q: Is the closest building/woods to the broken unit in L3 the Wooden Building in M5 (3 MF; the blue path) away or are both the Stone and Wooden Building equally at 4 MF (the red paths) for routing purposes since the broken unit could not enter M4.