I don't think the Zombie-via-possession is the way Meph wants to go, Deth. At this point in the game, it might be best for Bracken's player to take control of one of the survivors in the inn, most likely one of the two guards after he croaks.

Something I might offer as advice in regards to the xp problem for keeping the characters low level but giving them rewards could be to break the gestalt into a more granular system where you advance from 1//1 to 2//1 then to 2//2
Something like:
Spoiler
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{table=head]XP|Gestalt A|Gestalt B
-500|1|0
0|1|1
1000|2|1
2000|2|2
4000|3|2
6000|3|3
9000|4|3
12000|4|4
16000|5|4
20000|5|5
25000|6|5
30000|6|6[/table]

It'll inflate the value of XP a bit since it takes more XP to get to the same class features but you could always change the XP values to fit the rate at which your characters progress.