1) No. It is a very low-aggro spell, so if you use it immediately during a fight where you don't have time to gain aggro, then the mob will attack your fiend. However, it is extremely east to pull off of the fiend. Basically, don't worry about aggro with this one. It's pretty much there to give you mana and about 1k extra damage.
2) Yes, by far. The mana cost is relatively cheap for me, and every hit alone gives back more than it costs. For me, it costs ~200 mana, and each hit gives back ~300 mana. It's basically a free mana bar.
3) Again, yes. It deals shadow damage, and its damage increases with yours, much like a warlock's pet. More spell damage on you -> More damage by your fiend -> more mana for you.
4) It's a viable anything spell. Just pop one when you're low on mana, and you'll have mana coming out of your ears in a matter of seconds.
I basically use it as on "OH CRAP" button when I overpull or am low on mana mid-fight.