Would be fun make the same recipes from different editions and compare. I've noticed some minor variations between my parent's copy and mine but neither are really old.

I remember my ANZAC biscuits recipe in particular has less butter and more sugar, like you say, and is clearly inferior. A bit sad. However we also have Edmonds: Best of Baking, which is newer than my Edmonds Cookbook, and prescribes more like the old recipe with more butter.

I remember looking up eggnog recipes and realizing that the 1950s used twice as much whiskey as the modern versions

That was some good eggnog

Looking at recipes on the Internet it seems eggnog barely contains any whiskey by volume. By comparison German Eierlikör consists mostly of korn (a 32% ABV clear liquor similar to vodka) or white rum.

I'm not sure what 1950s eggnog looked like but I'm guessing it was more like the German style Eierlikör (i.e. made mostly with liquor and eggs) than the modern eggnog (i.e. mostly milk and cream).

Was the whiskey of the 1950s lower proof?

Definitely not. In fact, I think more of it was probably bonded (100 proof).