Even though they set up the trip to the lab as going somewhere to discuss Barnabas' treatment, quite obviously the real reason is to show vampire mice and to have Barn attack and kill the cleaning lady.
Julia and Barn will, however, have their discussion of his treatment down the road in this issue, and in a location that's always made perfect sense in every version of DS, so it only calls more attention to how needless an excursion going to the lab is. And what's interesting is that, unless I'm forgetting, I don't think they ever go back to that lab.
As for jumping the shark, well, just wait because IMO there's an upcoming plot development in Book One that jumps a lot more than a mere shark. Thankfully, though, as things progress there are other aspects of the plot that are quite interesting and almost make up for it...