Good video, thanks!
The weakness that I see with homomorphic is passing the K to the person you are sending the BTC to.
If you send the K by email it must be encrypted for example with PGP which we are not really sure if it has a backdoor or not and you would also need the public PGP key of the receiver.
The idea of using the invoice number passed through for example SHA-2 I belive is also weak because in that case the merchant has to pass the invoice number to the buyer so it is the same case as above but from merchant to buyer instead of buyer to merchant, and if the invoice number is displayed in a https website we know that the SSL stack is compromised...