He wouldn't have been sucked into a Pocket Dimension because that concept has already been debunked and laid to rest.
Anyways, time would still see to him still existing in 600 A.D. to summon Lavos and not get sucked in. If not, then he would suffer from the Grandfather Paradox (at least, that's what I think would happen).
All I know is that if Lavos was destroyed in 12,000 B.C. by your choosing, then that would entail him never being sent to 600 A.D. If that was the case, then he would've never summoned Lavos, and the party wouldn't be there to stop him, and they never would've caught up to him in 12,000 B.C. to stop Lavos in 12,000 B.C.(ad infinitum)
Hopefully, this helps to clear things up.