Great series you have going here, love me some good sci-fi with plausible technology. You explain things very well!
Just a few ideas:
1) borrow some robots from the space station for exploration of the doors, assuming they can go through doors, or get upgrades to the computing level of Difris or Nyf. Have them come along as security for the trio of humans as they travel.
2) more 'mini-minds' like Nyf and Brik. Each human should have his own 'buddy'. Even make them some bodies, maybe not as big as the other robots, but more scout sized.
3) send a wired connection through the doors as they explore, giving a live feed back to Difris.
Anyway, enough of me hijacking your ideas. Looking forward to sequels. I think I'll dive into the Charlie Boone stories now