More relevantly, nothing stops you from creating a tradeable note that says "redeemable for 1 bitcoin from <trusted authority>" and then using those as currency.

This happened with gold, and also would happen if we saw a true bitcoin bank rather than bitcoin wallet -- they'd pay a bit of interest, and in return would pool their deposits and use some fraction of them to generate revenue through other channels.

bitcoin-B would not be bitcoins, nobody would use them, and they would at best be tulip bulbs.

Unless you are using them to do something different then currency.