The quick filter not being installable in Astrape is intended. It specifies that you have to be able to "get at the internals of the recycler" to install the quick filter. This means you can only install it on mobile recyclers (the ones found at Laurion or Santorini which can be loaded on the rover), and not on recyclers which are built into a facility.
The coordinates you go to near the tunnel are a one -time visit to check out what was there. You learned some info which will be useful in future content, which you can review in the Notes section of your PDA. Once you've been there, the only reason to return to the tunnel is to get the power cells from the rover.
The expected price for the steam version will be £5.