I stir whilst mashing, about once every 15 minutes. I do it to ensure I have a consistent temperature throughout the grain bed. Once I begin recirculating and sparging I do not stir...
I might recant my earlier statement...did you use a yeast starter? And what was your final volume prior to pitching and about how much headspace was there? How much did you aerate and how? I've had plenty of instances where a blowoff tube was needed, but I've never seen anything this bad before...
Blow off tubes don't absorb the pressure, they just redirect the excess gas. If there is more gas/kraussen coming through the hose than it can handle it will be released elsewhere. Basically, it's the path of least resistance...
EDIT: In the future get a bigger blowoff tube or a larger...
It's not over active it's healthy fermentation. Your little yeasties are tearing through the sugars. This is a good thing. Just maintain sanitization and you will be fine. If you are really concerned look for some threads on here about "open fermentation." Lots of people don't use lids and it...
Is the keg chilled? Most of the yeast should settle at the bottom of the keg in a day or two. The first glass should get most of it out. And how we're you planning on filtering yeast out? I didn't think diatomaceous earth had reached the homebrew market yet...
I don't think it would have much of an affect on the spices. I'm not a chemist, but I know whirlfloc coagulates with positively charged proteins (break material). As long as the spices you are using aren't "proteins" then I think you'd be fine.