I don't see how the true extent of radiation-polluted territory would have affected the decisions of the Terra Novans. For all they knew, the entire planet was polluted; certainly this would be the case if they were under attack by outside intellects (evil Earthlings), as those would not leave a way out for their victims. So, no attempt at walking out, ever.
As for checking out on Terra Nova, the problems would not be solely logistical. Quite possibly, Vulcans would count trips to Terra Nova as "deep space" sorties, and would deny permission from humans - going that far out would incur the wrath of Bug-Eyed Monsters and other things humans were not ready to confront yet. We know humans found colonization targets much closer to home eventually - first at Mars, then supposedly at places like Alpha Centauri - and those would both reduce human interest in the fate of Terra Nova and allow Vulcans to claim that no real curtailing of human spaceflight was being done.
Also remember that this was all covered in dialogue. The colonists themselves said they wanted no checking out; the Vulcans would have done it for Earth, but Earthlings knew better than to ask for something that carried an unknown price tag; and with these things given, the nine-year trip there and back simply wasn't worth the effort.