As for boosting the power of Doppelgangers in small Towns, I think Doppelganger specific Modifiers would be a way to go. They're really missing a bit of passive power to their roles when they kill. Something that's one-shot is gonna give them more power without breaking the game too hard. Like, for instance, giving the better Active role a one-shot Kill Immune gives incentive for the Doppelganger to kill, as opposed to just giving it to the weakest power role. Heck, giving each of them something different that makes them something more powerful than a normal Townie might be better. Two Dopps with these powers can fight stronger Towns, so coming across a strong Power role wouldn't be out of place in those cases.
This would mean coming up with some, but fear not, as I have ideas. For balance and player reasons, I think telling the Doppelganger when these are used is for the best, and purposefully not using them should also be a choice:
Bone Plating
One-shot kill immune
Flavor: This Doppelganger had issues merging with the Human correctly, but bits of their skeletal system were hardened as a result.
Caveats: Remove Tough Doppelgangers. I think this would be a better way to go.
Hunter's Camouflage
One-shot Tracking immune
Flavor: This Doppelganger was a hunter from birth, and maintains a bit of its Doppelganger skills and instincts even still. It can change its human's skin color, height, weight, but too drastic of a change and it's gone forever.
Reflexive Shapeshifting
One-shot Inspection immune
Flavor: This Doppelganger was particularly adept at changing forms, and was particularly cunning. As such, the first time this Doppelganger is inspected by an Agent or Detective, it can choose to be a specific role and race.
Calm Mind
One-shot Psychic immune
Flavor: This Doppelganger doesn't resonate well with their human, creating a psychic resonance that makes it hard for Espers to even go near them.
Caveat: This is like a weaker Mind Shield. Or, it could be a one-shot version of the stronger Mind Shield too.
Stalker
Appears to use the same action as its kill target
Flavor: Since this Doppelganger stalks its prey, it can appear to use the same physical actions as its target while hunting.