1- i can not answer this question because i did not use "refer a friend"

2-not sure if there is a topic for boosting, you can check the wiki or use the search
I use a prot pally to boost my lower lvls, but i assume a dk will do fine as well. your goal is to park your lower lvls in the instance and grab as many mobs as you can take on your 80 dk, bring them back to your lower toons near the entrance and kill them for the xp. then when the path is clear you can take your toons further in the instance and do it again. I found some instances you can not train the mobs too far from their spawn spot or they will drop combat and run back to their spot. not sure if this was fixed or not. As for abilities you can just use "Death and Decay" in a group of mobs and they should go down pretty fast in lower instances.

3- pretty much all classes are easy to boost. when you get to higher lvl instances, being able to heal your group and what not is great. but for the most part your toons will not be attacking or casting any spells just incase they get aggro from doing so. not sure about high lvl instances though.

4- depends on were you go to boost them. I am not certain, but you want the lvl of the instances to be higher lvl then the characters you want to boost. But I could be wrong and someone will have to correct me because I usually just quest on the lower lvl toons. as for how long it will take, it will take more then a few hours thats for sure. an exact number of time can not be determined because we will not know how fast or slow you will take the toons through the instances. I think I seen a thread stating someone got their toons to 58 in 7hours or less just boosting.

someone with more knowledge with dungeon boosting will have to chime in and correct me