I think the reason Lea was able to enter Harry's apartment in Changes was the debt Harry owes to Mab, who had ordered Lea to the apartment. In theory Mab could have entered as well.
As for Butters being safe behind Murphy's threshold, I think Harry might have been, um, stretching the truth. I'd bet that Erl could have broken in, but Harry was counting on the fact that he wouldn't go to the trouble (Chez Murphy probably has a substantial threshold after being a family dwelling for nearly a century) when he could just go about seeking other, more interesting prey.