If I added 'lifelink' to Duskmantle Guildmage, would paying the ability cost and then casting Mind Sculpt gain me seven life?
No, sorry. Lifelink is only applied when a source deals damage. Duskmantle Guildmage’s ability makes a player lose life, but that’s not the same as dealing damage. (Damage usually causes life loss, but they are not the same thing.)
Okay, what would happen if a Volatile Rig dies, triggering it's ability to deal 4 damage to each player and each creature, if it has lifelink? Do I gain life from the damage it dealt? Or does the lifelink become redundant upon it's death (if it was enchanted with lifelink for example)? What if it was the only creature I had in play while controlling Homicidal Seclusion? I hope you can help, sorry for the difficult problem!
According to the MTG comprehensive rules,
702.14c If a permanent leaves the battlefield before an effect causes it to deal damage, its last known information is used to determine whether it had lifelink.
702.14d The lifelink rules function no matter what zone an object with lifelink deals damage from.
The bit about “last known information” means the game essentially takes a snapshot of what the creature looked like just before leaving the battlefield.
In this case, Volatile Rig had lifelink when it left the battlefield. When its ability resolves, if you lose the coin flip, the damage the ability deals will cause you to gain life.
(P.S. — Homicidal Seclusion is one of my favorite cards. Kudos for asking about it!)
You're not alone
When one of your loved ones discharges from the hospital, and you have no idea what post-hospitalization care’s about, you need not worry. Take some time to have a look at this http://lifelinkhomecare.tumblr.com/post/5693953077/lifelink-homecare-is-one-of-its-kind-in-the#notes .