Edward,
I cannot think of any way to get a click track to automatically ritard the way an actual performance is doing. You’re lucky if you can get a click track in sync with it before they start any tempo changes, I think.
I don’t find Musescore playbacks to be too offensive. I think they are fine for practice (more than fine), especially as far as your recorder intonation (which I have a lot of trouble with). Also, for complex music like pre-Baroque, creating playalongs for practice is a valuable tool, I think.
Also, with a Musescore playback, with you playing along it can sound fairly good, I think, at least if there aren’t too many places where some other instrument (played by the computer) takes an extended solo.
I listened to some of the tracks in the Continuo Lines pieces I have (from a collection called “Florid Adagio”) and the harpsichord is playing both left and right hands. I agree that the realization is fairly simple, but not objectionably so, and fills things in nicely.