Some years ago, before had access to the net, I asked MMP (or was it AH?) a "written on paper" question about shellholes in other terrain as OG.
I remember the answer was the other terrain still existed (i.e. grain will go on being a hindrance).
Now, I don't remember the MF part of the answer.
If one is a "rule lawyer" one must stick to the 1MF/2MF entry choice, whatever the terrain.
And as the rules don't say that the shellholes make the other terrain disappear, one applies the other terrain's rules (excepted the MF cost that is precised in the shellhole rules).
Note that if the other terrain remains, a Flame (and a Blaze) can be created in the hex - that is when the 150+ OBA creates a shellhole with a KIA (there is a reasonable possibility of fire beginning in the hex too).