I see no problem keeping them as NPCs- as we're heading off on our journey to the new continent, its not wise for the entire warband to venture off on their own each time- they'll need base camps, supply lines, and the people to protect them- as well as the ship that they arrive on, as that is their only way back. And who better to protect these key positions than a small detachment of Inquisitors from the warbands? Galahad as the warleader would probably find some value in keeping at least a few Inquisitors in reserve while the rest venture forth, especially in these hostile lands.

Hassan is very versatile in social situations, but while he is an Inquisitor, with all the training an Inquisitor has, he's not nearly as frontline a fighter as Viveca, Ziotea or Stina. Makes sense for him to stay behind and work with the non-Inquisitors and coordinate their efforts.

While Astraea is certainly a fighter, she also works very well as a healer, so having her in their base camp would make sense, to aid them with any injuries they may return with. She also doesn't work very well with others- particularly non-Lanostrans, or even other Lanostrans like Tatiana, and through their bickering while in Lanostre, I can see why Galahad might put her in the back, though it is a bit of a stretch. 

Rodion is a scout. Scouts work best on their own or in small groups, where they leave the smallest footprint and aren't easily noticeable. Having him perform scouting duties would be a good way of keeping his character relevant, but 'out of scene' when the rest of our characters are doing stuff.