We have a bit of complication that arose after I made an outright challenge to detractors who claimed Trigger was rife with plot holes. At first I thought this may be covered by warmgun's 'whose past no longer exists' axiom, but that only applies to memory. Here's the juice:
I have a bit of a theory on some of the events that happen throughout Chrono Trigger and Cross. I think that for all purposes of having an enjoyable game, there is time that sort of transcends time in the sense that things like Marle disappearing, or the Chronopolis appearing in the past don't happen instantly like they really should but rather, take a certain amount of time before they occur. I also noticed this when you defeat Lavos in Trigger the proper way, then Doan and Robo still exist, even though the future should be changed instantly and they would disappear. Obviously they used this so that they could stretch the rules of time travel.
The Marle paradox can now almost entirely be explained by the Entity; although this may externally appear like a case of gods transcending science, the reasons for the Entity's detaining are clear and purposeful.
My answer to the above problem: let us work under the Axioms & Corollaries definition of timelines. If the heroes of time battled Lavos in a time period other than 1000 A.D., returning to that period would mean Keystone T-1 would have instantly transpired and become Keystone T-2, as the heroes were temporarily displaced outside of time, allowing it to flow naturally. This would suggest that perhaps Doan and Robo do exist in Keystone T-2's future.
Unfortunately, this is traveling out of bounds. It is generally presumed that Squaresoft did not have Cross planned out, which is especially supported by the vagueness of Radical Dreamers. Thus, the 1000 A.D. Crono returns to is Keystone T-1. This suggests that the Rise of Porre did not occur in Keystone T-1, as Doan still exists.
Unfortunately, this is also traveling out of bounds, and the future was most likely not planned by Square. My only justification for this that I can offer is that despite the Rise of Porre, the Guardia line continues and Doan is able to live.
Unfortunately, for the Guardia line to continue, Marle and Crono would have to live -- however, they are dead in Chrono Cross before the dimensions are restored, an event that succeeds 1000 A.D. Our only shred of hope may lie in the fat that Crono, Marle, and Lucca are only said to be dead in Home World, where they reveal such on Opassa Beach prior to fighting the Time Devourer. If Another World is the true timeline, Marle and Crono may have lived and been able to continue the Guardia line, to the delight of those who assert Guardia retained its status as a country or province in spite of Porre's militarism.