I like this the best. Unless you're giving a standing invitation (as The Mighty Buzzard said). When I welcome people over to my house, I invite them in every time. There are certain people, like really good friends and extended family, who can walk in without the formality of being invited in. I think you can also withdraw invitations as well.
I'd go with this, adding the fact that Harry doesn't have to invite Thomas or Molly in every time. I know he has a shitty Threshold, but these people are closest to him and that is what counts. What just hit me, though, is the fact that they both have keys and means to undo the wards, so I guess if you give out your key to someone or something, with intention, then they should be able to enter until you retract the invitation.
I would compare it to inviting in someone on business, like the pizza boy or the plumber ('My, what a large tool you have...') Anyhoo... they do their job and leave. They won't just come back in without an invitation. Unless they're creepy serial killers, but that's besides the point. I feel intent counts.