Hey!
Sorry been busy lately. Really sad, that no main RK players could answer your question :/
1. I think it depends also of your Cast Time and your After Cast delay. Maybe I am wrong, but Parrys delay is shorter then that of Guard + you can Auto Melee attack without breaking your attack unlike Guard. It shouldnt be that of a Problem, if your Cast time is high enough, just for mobbing purpose I would deactivate it. It would be great, if a main RK player could say something about it, but I can test it out on Weekend and PM you or answer here.
2. It will reduce it. Use 2Hnd Quicken, if you are using TGL. The Animation of Dragon Breath is effected by ASPD. Its like trying to spam Bash, Brandish or Bowling Bash without ASPD or even worser, on a mount without Cavalier Mastery (on Peco, Gryphon) or Dragon Training on Ferus. Or try 1 AGI Build on your RG on your Gryphon with no Cavalier Mastery and try using Cast related skills like Overbrand or Moon Slasher. The HIT and Animation delay is freaking annoying.