Destroying Captured Planets

I was just wondering if a Warlord gets credit towards his win condition if he destroys a home planet that has already been captured by another player.

For example, if a Trader captures the home planet of a Researcher and eliminates that player from the game, can a Warlord still come in, blow up that Researcher's (formerly) home planet and get credit towards his win condition?

Yes, as it is still considered a Home World. The game remembers where everyone's Starting HW is, regardless of ownership.