I noticed that Tantive IV triggers "before you gain a command token."

Does this mean that a Tantive already holding a command token can pass a new token (Comms Net, Hondo, Dial) before having to discard the old one? Can this be used to chain tokens to otherwise ineligible targets (Hondo giving 2 tokens to 1 ship, comms net chaining to Distance 10 etc.)?

That is the point of the difference between Tantive and Comms Net, yes.

On the second point, yes as well. Hondo is technically assigning to the Tantive Vette, where it sends it is irrelevent to the Hondo Ability. If Hondo has already sent a token to that target, so be it.