No saving throw against the dispel; I'm pretty sure no dispel effects in the game use that.
Carsomyr's on-hit dispel is level 30, as is the Staff of the Magi. Arrows of dispelling always dispel with no level check.
The planetar weapon dispels with "use caster level", with a 26% chance coinciding with the vorpal effect. This is not changed by SCS. Planetars are level 25 mages. There's a parameter where the level goes for "use specific level"; zero in vanilla, 15 in my old SCS install. (Edit: that's not a mod change, that's a 2.6 change. SCS does not modify the weapon at all.)
Testing it (cast Death Ward on the Jailkeep Golem, tell planetar to attack it)... in vanilla, a level 12 Death Ward stands up to the planetar's dispels for a while, but not forever. Seven dispel/vorpal hits, three failed saves, and it went down. The SCS version was about the same.
Further testing, using a party member on my SCS install: The first vorpal hit fails to dispel either a level 8 Stoneskin or a level 12 Death Ward. The second kills the target.
So ... the ordering doesn't let Death Ward protect against the vorpal effect if the dispel check succeeds (I'd change this, and move that dispel effect to the last spot). But on the flip side, it's definitely not a level 25 dispel; the dispel check fails way too often for that. I don't know what level of dispel it is, but I suspect around 10 (with or without SCS).