[Galemp]

How many GTA assets (mainly exploration groups, science vessels, possibly installations) were present beyond Ross 128 (say, Luyten or Laramis) about a year prior to FS1?

As many as you need to tell your story.

And how much creative license do I have in that respect?

As much as you need to tell your story.

I'm going to take the Dungeon Master approach here, and say that everything here is nothing more than a guideline for you to follow so you can make the campaign you want.