I'm planning on boosting to 88 herb to make extreme attacks during bxpw. If I log out every 60 seconds or so, to keep the boosted level up, will the 2.7x multiplier still decrease? If so, will it continue to go at the same rate?

It will continue to go down just as fast as normal.

That's not true-- I've kept the 2.7x multiplier practically the entire time every weekend that I've had to stew for herblore

It will continue to go down just as fast as normal.

 

Needs clarification: it will go down only when you're in-game and playing - which is as "fast as normal".