QUOTE
R. Butler... why is mom 100% owner of ABC? (at most)
Lets assume Mr. D is under 21. It is pretty safe to assume mom is over 21.
Mom is attributed 100% of ABC. She still only owns 50% of XYZ. Nothing will be attributed from granddad because mom doesn't own more than 50%.
Mr. D is deemed to own mom's 50% of XYZ, but again nothing attributed from granddad to Mr.D because Mr. D doesn't own more than 50%. The fact that Mr. D is under 21 is irrelevant in determing attribution from a grandparent.
So at most we have 100% ownership in ABC & 50% in XYZ.