I just started reading the Bullywug Gambit even though we are not done with There is No Honour. We have one spellcasater who specifically focused on illusions and mind-affecting spells. He's already frustrated by the zombies and he role plays well enough that I want his powers to have an effect. Is there some other immunity I could give to savage characters while allowing mind effects to work? What would you suggest?
Also, what level would you recommend for the start of the chapter?
Just get rid of mind-affecting immunity and don't replace it with something else. The savage template is a bit overpowered for just +1 CR, so this change should be fine. If you feel the need, you can give the savage template a +2 save bonus to charm/enchantment.
Level 3 is the expected start point for the adventure. The assumption is that the PCs have a lot of healing potions that they recovered from the Lotus Dragons in the first adventure.