But - neither deepest diversity, nor first branch split can indisputably tell us about place of split.
For example, it turned out erroneous to apply same logic and look for PIE homeland in Anatolia (since Anatolian split first).
Also Kallio’s idea on late (2nd century) arrival of Finnic into Latvia comes from linguistic not archeological facts. Basically he wrote that North Latvian hydronyms can be explained via Estonian or Livonian not Proto-Finnic.
How is your community of Finnish linguists? Are you like pals who can chat with each other or more like formal. Like I could ask any of my colleagues informally about everything. Could you ask him directly what he thinks of current evidence for Southern Route?