Tipsix should be in the cave (just south of the farmlands, west of the Blue Boar inn) after you talk to Huge, even if you had found the cave earlier.
It doesn't matter when you confront the doctor the first time. Even if you had found the room he rented full of poison, he would just have claimed to have lost the key to the room long ago, and didn't know anything about the poison.
IIRC you need level 2 Lockpick for Elrath's house, until you confront him a second time after entering and getting hard evidence, after which it is not Lockpickable. The house key is a bit south east of the house.
