Switching tires

Well looking at the cross reference info I can find the Venture uses a 15 X 7 Wheel with a 40mm offset and a 5 X 115mm bolt circle. The Mercury on the other hand runs a 16 X 7 rim with a 42mm offset and a 5 X 108mm bolt circle. So the lug pattern doesn't actually match, the center hub is a different size and the rim is 1" smaller that you want on the wagon. The item about the brakes is that the 16" rims just barely clear the brakes used on the Mercury as it is a wagon so it has the larger brakes than the sedan. That means the 15" rims are not going to clear the calipers.
Sorry to be a bearer of bad news.

Been there myself, old car had new 16 inch pricey snows, New car takes 17 inch or optional 19 inch. Sold old car to a young lady who totaled it about a month later, went to the scrap yard with new tires, made me sad.