We could argue that BRS has distorted the South Wales market over the last decade with EZY and FR, attracting so many over the bridge
You might be able to, I can't. Bristol has not distorted any markets; it has just outperformed its neighbours by dint of having a more affluent population, and larger catchment. That's not market distortion.
I'm not at all sure that Bristol Airport has incentivised agents not to sell Cardiff. At the end of the day, the airport does not sell seats, or holidays. If the airlines are doing any differential pricing, that is an issue for them.
Oh, and you'll find that devolution of tax powers certainly is a legal matter; it will almost certainly need an Act of Parliament...
TA