I've just read the Coiled Serpent PP in question as well ... and it doesn't seem to be so problematic. The Frost/Lightning weapon seems to solve most of the problem.
Unless I'm missing some loophole where PP benefits also add keywords to powers (but I'm guessing this is not the case. Guessing. Repeat Guessing
)
The weapon's at will power says " ALL damage dealt by this weapon is cold damage ... "
So ...
Serpent form Feature ... gain resist poison 15 in beast form and +5 stealth: Great, not affected whatsoever by undead or non undead poison immunity.
Snake Fang Action: AP used to make an attack, each target hit takes ongoing 10 poison damage. This does not mean the power in question gains the poison key word (as far as I know). So the powers effects remain effective vs undead. And If the weapon is your implement used to make the attack, one might argue that this extra damage is also dealt by the weapon, and hence it is also changed to cold. Hehehe ... I'm sure DracoSuave will have more to say on whether this is legal or not
lvl 16: Serpents Poison: Hit with melee attack in beast form; deal 2d6 extra poison damge. No mention of gaining the poison keyword. Again your frost Dagger that has melded with your body in beast form and become frost claws also can convert this damage to cold damage, as All damage dealt is turned to Cold damage. Right?
So the features seem to work.
lvl 11 power: No poison involved at all. All good.
Lvl 12. No poison. All good
Lvl 20: Fazing Fangs. Poison key word. so you'll do the dmg (turned to frost) but no effect (no dazing and hence no damaging them if they attack. Shrug ... surely you don't fight ONLY undead creatures ... Let this one rip as soon as you come up against non-undead enemies. Besides it's only one power.
WOrst case scenario, if DracoSuave throws my suppositions to the wind, then you seem to be gearing towards Sriker Beast Form type druid. So Blood Moon Stalker is not a terrible second option. No control whatsoever unfortunately.