... but even before the rule change, in virtue of being a proper name, if the proprietor calls it "Eva's Blumenladen", and it's marked as such, wasn't it proper usage to refer to it that way?
If I call my English business, "Joes Cafe" (intentionally not using an apostrophe), wouldn't it be incorrect for people to refer to it in writing as "Joe's Cafe"?
Absolutely. You don’t need to come up with fake examples. Take a couple of high end British retail establishments: Harrods and Selfridges, founded by Messers Harrod and Selfridge, and neither styles itself with an apostrophe.
Define "proper usage". You can call it anything you want, but if you call it "Evas Blummenladen", people will wonder why you spelled "Blumen" wrong. Now sometimes people do that in names as a play on words etc., but here it just doesn't make sense.
In the same way, people will wonder why you spelled "Eva's" in "Eva's Blumenladen" wrong, if you spell it that way.
Yes, if enough people start doing a wrong thing, it'll eventually become "right", and I guess in 100 year's we can put apostrophy's where'ever we wa'nt, but currently it still looks odd, and like something that is foreign to German, and imported from English. Because unlike in English, in German this apostrophy doesn't stand for an omitted letter in the genitive singular ending.
"Eva's Blumenladen" is the proper name of the shop, what is put on the sign above the door.
"Evas Brille" is just Eva's glasses.