1) Do Changelings have to follow the hospitality rules if they cross a threshold uninvited? I would guess not since they're still part human and have free will. But, then again, Wizards are human and thresholds affect them.
I think you're conflating two very different things, here. A threshold will affect their magic, the same as it does for any other magic that crosses without invitation. To be fair, Changelings and other Un/Seelie, et al, that are powerful enough to do this in the first place are likely to be mostly unfazed by anything short of VERY powerful thresholds.
However, being invited, or not, has to do with the Accords. Inviting such a being into your home DOES allow it to bring it's power with it, but by accepting the invitation, they are bound, on an almost physical level, by guest rules. To the Fae, in particular, these are as potent and binding as any loss of power might be (for entering uninvited), possibly moreso. As a guest, they are BOUND to treat their host with at least some courtesy, presumably including the implication to do no harm.
That being said, there's nothing preventing a crafty fae (and what fae ISN'T) into trying to goad or trick their host into breaking the similar HOST obligation... leaving said host entirely at their... mercy...
As for other beings, they can follow the Accords, or not. As long as they can cope with the consequences of the latter choice, the host is fair game. Keep in mind that even Mavra, as powerful as it is, has proven reluctant to break bargains, though I can't think of an instance where it was invited into someone's threshold. And it's not nearly as beholden to the Accords as the fae are.