The 10th Mountain, as other formations from ETO/MTO was not slated for Kyushu (Operation Olympic) and not even in the scarce planning made for the invasion of Honshu. While it is possible that the division would have been shipped to the pacific as follow up I have not seen it mentioned in any troop allocation for downfall. Indivisual troops from ETO/MTO formation would have been transferred, even formed squads, but I do not have information about the 10th Deployment to the pacific. Is possible that your grandfather was to be transferred out from the 10th to another unit?
Arrigo