1. I think you'll have to play with the water level, my friend had a pumpkin, and if he filled it .5" above stem the water would come through the hose.. :/
2. Can't find any review of the Taru Quicklites you mention, but if you can get natural coals(which i would, they are usually better) then yes, you can light them on an electric stove.
3. That depends on how hot your shisha is getting, its a tool for heat management. Experience is going to be your best judge. Shiny side down
4. There should be brushes for hookah's at hookah retailers, not sure if they are small enough for a pumpkin though. You might try pipe cleaners, and a tooth brush for the base. Normal hot water works most of the time, if you're really looking for a deep clean I would recommend toothpaste as a cleaner, and if you dont want to use that lemon juice..