Manda, there is some reasonable logic behind your points. And I don’t believe there is ever going to be any clear-cut answer on this. But I do think there’s another angle to this that you should weigh when considering the question: namely, that even if it may make sense in a technical legal sense to talk about “the grandson’s costs”, in reality there are no such costs for “the grandson” (my son–let’s call him “Son” in honour of the Michael Shannon movie I just watched on Netflix). His mother and I are legally (and, at least in my case and probably hers, morally) committed to paying all college costs.
Therefore (recalling that 2X is the total cost of college, and Y is the amount in the college fund):
(1) If my mom was just lying these college funds all along, my ex-wife and I each pay half, and Son goes to college without either having to come up with any funds to do so, or incurring any debt. My net worth is reduced by X, as is my ex-wife’s.
(2) If the college funds are real, and they are “taken off the top” before my ex-wife and I split the rest, Son still goes to college without either having to come up with any funds to do so, or incurring any debt. My net worth is reduced by (X - Y/2), as is my ex-wife’s.
(3) If the college funds are real, and they are instead applied to my side of the ledger, Son yet again goes to college without either having to come up with any funds to do so, or incurring any debt. My net worth is reduced by (X - Y), while my ex-wife’s net worth is reduced by X.
So if we get away from technicalities and judge this by what actually happens when the rubber meets the road, this college fund really isn’t impacting Son’s finances at all. It becomes in essence a partial reimbursement of parental outlays: either equally reimbursing her ex-daughter-in-law and me, solely reimbursing me, or reimbursing me more than her (if we go the route, which I again would consider fair, of treating money contributed before and after we split up differently).
And while, as I say, she is not likely to want to go on the record with a specific stand, deep down I feel sure she she is no different from the typical person who is more partial to giving their money to their offspring than to a former in-law.
ETA: Want to quickly add during the five minute edit window that I composed the above after only reading through post 42. I will now read the two posts that popped up in the interim, and add a new comment below if needed.