Thats right, David Brierly was the voice of K-9 for Season Seventeen and even then he did not appear in every serial.
I was reading an article on Shada and JNT tried several times - even after Baker left - to remount it as a special so that the story could be completed. Brierly was contacted about recording new lines as K-9, even though Leeson had come back by this point, for continuity reasons. Sadly, the actor is now deceased, so he wouldn't have been available to do the Shada audio book (he was around back in 2003 when they did the webcast, though, but they used Leeson for that).
Nope that's correct the only Romana 2 Target story done to date is Creature From The Pit, which Tom read.
I get the feeling they're recording these pretty much in random order - maybe in the order they get the rights or something. With Shada now published all eyes are on whether BBC Books gets the greenlight to do City of Death next, so there might be another chance for Lalla to do that one. (Hopefully there'll be a book and not just an audio as is the case with October's release of Resurrection of the Daleks.) I looked ahead and the next Target fourth Doctor release is Androids of Tara in July and they haven't announced who'll be doing that one (hopefully Mary Tamm since it was very much her story).