It's not directly answering your questions, but I do like .pkpass. It's a good standard because it does not require accounts, links or anything like that. If you have the file, you can import it. It's the opposite of how Google does it, which makes it useful beyond Apple.
PDFs/Screenshots being invalid because it can be sent quickly to multiple people is a bad reason as you could print out multiple copies too. Sadly, being sensible is so often not a thing.
FossWallet seems to be great although I didn't try it yet.
I also really hate it when services restrict downloads depending on the platform. I.e. it could be possible that Apple users get shown a .pkpass, while Android is artificially limited to Google Wallet - even though not all devices support it. Maybe setting the user agent could help but that's just a wild guess.