An animal companion takes the Ability Focus monster feat (Choose one of the creature’s special attacks. Add +2 to the DC for all saving throws against the special attack on which the creature focuses.) and this feat is applied to a poison ability.
If then that poison is harvested (Via Milk Venom trick or otherwise), would that harvested poison retain the +2 DC when used?
I don't see any reason why it wouldn't retain the increased DC. When you use handle animal to milk the venom it just states that you get one dose of the creature's venom. There is no indication that it is better or worse then the creature you got it from.
That being said unless you have poison use, it looks like you'll have a 5% chance of being exposed to the poison accidentally. Additionally, the dose you milk will only stay potent for 24 hours unless you preserve it via craft poison.