I like that, but I would change it like this:
Original:
Parker and Hardon.
Thank you for the control codes.Unfortunately, they were of little use to us, as your surviving probes have been shown how to change their own codes and are, in fact, now our allies.
Comment:
The first two sentences are perfect as they are. The "thank you" is properly perfunctory and condescending. The statement that their probes are now allies takes the wind out of their sails.
Original:
However, you have indeed shown that you are trustworthy and we include images of some alien markings we have found. We are not sure if these images pertain to a current civilization, or one that may have disappeared a long time ago.
New:
However, you have shown that you can negotiate in an apparently fair manner. We are therefore including images of some alien markings we have found.
Comment:
I thought declaring them "trustworthy" was a bit much. It took far too long for them to recognize the reality of their situation. In fact, they could have concluded their entire complement was destroyed either in the failed landing or the battle that was later reported (as the rescue of Andrew) and giving those codes was a concession without cost. Saying "an apparently fair manner" says we extend a measure of trust but also retain a healthy skepticism. I removed the part about not knowing if a current or past civilization left those markings because we have been very careful to leave Parker and Hardon confused over that point and we should not provide an unwanted clue with that statement.
Original:
We will send you future information upon the release of Miss Dilyn Carter. As soon as this is done, we will begin to share what we may learn here.
New:
We agree to send you some future information upon the unconditional and safe release of Miss Dilyn Carter. Once we have verified she has been returned unharmed and no longer under threat from you or your associates, we will then be prepared to share future discoveries with you.
Comment:
I explicitly stated that new information would be from new discoveries. That gives us a legitimate reason to keep discoveries made to this point to ourselves if we consider them too sensitive to release.