On the final boss, the dialogue text gets cut off. And after getting the Agate Knife before the cursed seal, Merit's lines are completely gone from the new dialogue.
The save provided is after obtaining the Agate Knife.
I have Windows 10 and it worked fine for me, but that may not be the case for everyone. Although for me, when I connected it it didn't show up as Wario Mass Storage. It just showed up as a different SD card. No driver installed when I connected it.
Got mine recently. Everything you did looks fine. Mine was the same way when updating the firmware (would flash green once) but it did it's job. I have a Sandisk Ultra 32GB card in it. The fact that you tried 4 different microSD cards and all with the same result it could be what @nero99 said...
Even if it's possible, it seems more malicious than playful. It's actually editing someones work without their consent. Although just using it on a friend wouldn't be too much of a deal you would probably have to make it yourself for personal use be it in good fun or otherwise.