- The Spontaneous Mutation effects go up to 7+, but only a single D6 is rolled. Seems like there's a missing modifier of sorts? Any idea what that could be
The 7+ is from the End Times table, not from the Spontaneous Mutation's sub-table. Basically that just means, that at turn six every downed ganger starts to explode with Daemonic goodness. Yay!
There are seemingly just six variants of Random Mutations.
- Daemonic Incursion (7+) states that fighters can become possessed, but excludes the rules for the Daemon to jump to new fighters. "Note that unlike the Daemonic Possession scenario, these Daemons will not jump between bodies and remain in play until they are exorcised by an Underhive Exorcism or the battle ends.". What happens if the possessed fighter is taken Out of Action by something other than a melee weapon, or the 5+ exorcism roll fails? According to the rules of Possessed Fighters, exorcism is only when taken out of action in close combat / coup de grace AND rolling 5+. Otherwise it jumps.
Classic N18 rules writing, y'know what I mean?

RAW it's definitely unclear how exactly the posessed gangers behave, when they go OoA, however that's far from the first such instance, so TO THE HOUSERULE CAVE, I say.
Two ways to handle that seem obvious from the get go, maybe your group will come up with something else, but anyway agree before you try

1. The ganger going OoA is just removed from the board as normal, the Daemon doesn't jump and is for all intents and purposes also removed, and the ganger, who caused the OoA (if there was such a ganger) receives 1d3 XP in the end of this scenario (see XP reward in scenario description later). If the OoA was caused by a Melee attack or a CdG, check if the Exorcism is successful on a 5+, but in any other case the fighter going OoA still causes the 5" blast as per Daemonic Incursion result on the End Times table, so the Daemon effectively "jumps" in this fashion, provided there's somebody close enough.
2. The second way requires way more bookkeeping and is probably better for an arbitrated scenario. You can ignore any OoA results which are not also successful Exorcisms and stand up downed possessed fighters every turn, only giving bonus XP successful exorcisms. The 5" blast still occurs when an OoA is rolled as more Daemons breach the veil. This variant is way more brutal and, possibly more fun, depending on your preferences. Most everyone might end up possessed.