H - Q3/4 (hopefully you'll get the rest from Flaps' links!)
3) It degrades the performance of the surface due to its viscous nature which affects the airlow (not as much as ice/snow!). An adjustment is made to take-off performance to allow for this. Effect on landing/cruise performance judged negligible.
4) I have operated several flights, many over 1:30 long, where de-icing fluid is still evident on the surfaces at the end. It IS viscous! Regarding dissipation (the fluid, not me!) I guess speed and rain would do that more quickly, but I'm guessing. I cannot see altitude having much effect unless it gets so cold it freezes itself during the cruise?
Hmm! Some "wagon dragon", Flaps!
