I have not tried any of the revive runes.
I am not sure if Elixer S Trait and Magnetic Aura would proc at the same time. I guess it depends on how big that last hit that took you below 25% was. If it dropped you below 20%, I am betting both would proc. Aura lasts 5 sec against only projectiles. S is 3 sec immune to everything. So you would still have 2 extra seconds against projectiles if both procced at same time.
Rune of Undead will give much more Condition Damage than Rune of Engineer.
Sigils do not stack like that. Sigils with cooldowns all share the same cooldown, so 2 x Sigil of Ice would be redundant and you would only have the 30% chance. It would be best to go with either a stacking sigil or passive sigil for your second one.