Allow me to make some points here:
About the Ice cream shop I guess our biggest lead there would the FUCKING APPLE ICE CREAM, that is a really uncommon Ice cream flavor and I can bet my ass there are very few stores that do sell it.
Second the thing with the trees it's also a very good lead, that might narrow to an specific climate to look out for.
And lastly, the biggest lead of all of them is to figure how the "victims" look in any of the evidence where he participated, that way we could also look for previous adoption posts or missing pets, as well if we happen to find a potential suspect we can look for confirmation on photos or posts, people like this are prone to sharing but not as much as the average furry, they might want to show their new "friend" if it was ever adopted before making the final blow (no pun intended).
I try to figure how dense their families (if they have any) have to be, how they can't figure out how this bastard keeps losing any dogs?
EDIT: While writing this out I found that Great Value does make apple Ice Cream, so it doesn't even have to be "home made" just some place that sells it