I think it should be made more clear what's the reference price for the
discount.

That might be useful for merchants that already provide a series of price-differentiated payment methods, yes. Will think about it.

Also, currently PR are created by the payment processors afaik. How can
they know what other payment option the merchant provides and what's the
conditions?

Currently Coinbase let merchants specify the size of their discount (I guess in percentage terms, I should ask). So the merchants tell the payment processor. I don't think this is a worry at the moment.