If you’re leaning toward it being a convertible cookie, my first thought is you may want to go ahead and keep writing it in third person, that way new readers are getting a taste of what to expect in the related series. (Especially if that series is already doing well).
However, I’m also not familiar with the tropes of mysteries, so having a first person cookie might be fine (or even be exactly what readers would want). And with you leaning in closer to the shopkeeper, maybe it’d be fun to really get in and see the shopkeeper’s POV.
Or if first person is common in mysteries, then maybe that kind of cookie will help lead in readers who wouldn’t be as interested in third person, but will develop an interest in your world and characters through the cookie and go on to read the rest of the series out of curiosity, even if they might not have originally picked it up.
If it’s meant to be more of a cookie for existing readers, then I’d think you have a little more leeway to do whichever feels the most fun and exciting.
Good luck with the test scenes! Hopefully those will help you decide which version feels like the better decision. 😀