They changed/ clarified that passengers could only be activated to disembark to also being able to activate to do nothing. But can they be activated to do nothing while off table still sitting in the vehicle?

Where was this stated? I remember having that question answered officially a while ago, but was it published now?

Don't really see why the passenger couldn't activate before the carrier entered the board. I mean, throughout the match, the passenger can activate/pass before the carrier, so why not on round 1? The only thing is that once you activate/pass, you commit to being carried by the transport.

Q: One of my units is riding in a transport
and I don’t want to have it disembark yet.
Do I still have to activate it?
A: Yes. A unit that is embarked on a transport must still be
activated. It may choose to pass on its actions, but its activation
must still take place.

This is from the FAQ. So, I don't see why a unit on a transport that hasn't entered yet can't pass on the activation.