Sorry, I don’t generally check delving so usually miss replies, feel free to reply on the BIP PR.
I have not given it any thought, honestly.
Maybe? The protocol will always be fairly identifiable, including addresses which are going to be fairly easy to google and figure out, I think.
Because its already a well-defined format for communicating multiple payment instructions in a way that most wallets support. Why reinvent the wheel?
Because then a sender has to add extra logic to enumerate the set of supported formats and send multiple queries. Most senders already have logic to handle dealing with a single BIP 21 with multiple payment instructions, so we should just capitalize on that by giving them a simple API to fetch a BIP 21 to feed into their existing logic.
No such limit exists.