So I was doing some experiments with DI (T5 Warpriest /Warsoul) to answer some questions I had. Here are my findings:
1. Divine Intervention prevents instakill effects and death by negative levels. The ability is not removed immediately upon preventing death in this way (i.e. it'll last the entire duration). Regaining levels when the effect wears off does not cause death.
2. Anti-magic Cone does not remove Divine Intervention. The casting of the ability is not prevented while under the effect of an Anti-magic Cone.
3. Divine Intervention can be used under water.
4. Divine Intervention will not allow you to go past -9 even if your range of unconsciousness extends past -10.
5. Effects that take you out of bring you out of incap will remove Divine Intervention without proc'ing the healing effect.
6. Divine Intervention is affected by healing amp but not positive energy.
Number 1) was tested using Energy Drain and instakill effects such as Implosion and Slay Living. The person I tested it on had upwards of 100 negative lvls. I found out through this that hit points cannot be reduced by negative lvls below the lvl of the character (i.e. a lvl 5 character cannot have less than 5 hp as a result of negative lvls). I also used it on myself to go in the fiery pit in the Subterrane and lived (the one that is is used to open up A Vision of Destruction; still couldn't activate the button though). My guess is the effect prevents any effect that would put you past -9 even if it isn't a result of damage (and all instakill effects would bring you down to at least -10). Implosion did damage when a save was made but did nothing for save failure because save failure would have meant going past -9.
Number 2) was tested fighting a beholder in the Subterrane.
Tested number 3) in the Harbor under water.
Number 4) was tested with a barbarian with the Die Harder enhancement (Frenzied Berserker T1). Even though his range of unconsciousness extended to -25, the ability only allowed him to go down to -9.
Number 5) is an interesting case. This was tested with the same character used to test number 4). We found that because he got 20 temp hp when he goes incapped, he'd go back up to 11 hp WITHOUT receiving the healing portion of the ability. This in turns means that using it on barbarians that have the Die Harder enhancement may be more difficult to manage, since it would leave them with less hp than they would have from the healing effect itself (as demonstrated with number 6, the healing can be improved with healing amp). The ability is great to use in conjunction with effects like Reactive Heals (Radiant Servant T5) and Eternal Defender (Sacred Defender capstone) because the healing from them would greatly outclass the healing from the Divine Intervention ability itself.