I can't see how you could possibly know that they would become unavailable after you take the battle tower.
You are given a warning before entering the summoning chamber. Even if you don't think traders could be impacted, it is at least an obvious place for a save, and a relatively painless reload once you discover that the island gets gassed and the only NPC left outside there is the one who wanted the loot from the boat in a tree.
Besides, nobody gets everything on the first playthrough, anyway (unless they follow a detailed walkthrough).
