This question is a hypothetical based on the last play of Game 5 of the 2014 NLCS.
This has happened before, most notably the Grand Slam Single of the 1999 NLCS.
The Braves and Mets were tied 3-3 in the bottom of the ninth. The Mets loaded the bases, and Robin Ventura crushed a grand slam. He was mobbed at first base, and never touched second. Only Cedeno, the runner on third, actually touched home. Ventura was awarded a single, and the Mets won, 4-3.
Joe left a comment:
Once Ishikawa doesn’t hit a home run (because of him failing to touch home), only the leading run scores (as with anything other than a home run or a ground rule double, no runs may score beyond the winning run in the bottom of any inning at or after the ninth).