Your grinding/pve rotation is something like this:
sic Felg, CoA, Cor, Immo, tap or (tap x 2) to get mana, drain to up health - wand if needed.
Health Funnel Felg as needed. You can't go quite as long as a drain tanker between needing to have a little downtime and drinking, but it's easier to take on "bad situations" - like adds - harder or higher lvl mobs - elite mobs - and to take on 2 or 3 (or more) at once.
If Felg dies, you can pop out a new one in half a second. Just remember, he's not going to get agro off you on a mob you dotted up while previous Felg was alive. That mob is mad at YOU. So be sure the Felg is on other mobs (if there are adds in the fight) - so he takes them off you - and you deal with the one that hates you (or use Felg for dps).
The other poster is right - you want the talent that makes your Felg tougher, especially for pve (it isn't as needed for pvp, save that it helps you).
The talent that gives you +5% to crit is not needed for a drain tanker. I know that sounds like - what, it's awesome - give that up??? But I would consider not taking it if you are primarily leveling/grinding. You won't be casting shadowbolt or incinerate very often - it doesn't help much w/ dots (only helps immo - and it's a duel-edge "help" cuz it can pull agro on you).
I'd go into demon enough to get the Felg, at least 2/4 in Mana feed, the one that ups your spell damage to a % of demon's int and sta, and make your demon tough.
Then put the rest into Affliction. I think you can go as high as amp curse and nightfall by 70. But imp cor and uninterrupted draining are quite good. If you pvp consider destruction for shadowburn - but for pve, go affliction.