I'll give it a stab..
1.) You can leave the hose hooked up, but close the valves on the tanks and only dump when full. Reason being one, that sewer gas effect could go back to the trailer and two, especially on the black tank, you want as much liquid in it when you dump so it all drains out. On the gray tank, it's just nice to be able to flush that thru the hose after you dump the black tank.
2.) Probably for helping to control the smell.. I use them too. Just what works for me. Don't worry about getting the stuff that says it helps break down the solids... You won't keep the stuff in there long enough for that to happen anyway.
3.) No idea... :)
Good luck!
Mitch