I once sent a question to the WTS about this issue. I asked, why do you say that the 'seven times' started after Gedaliah's murder and NOT after Zedekiah's capture? Gedeliah was not a king. The last king was Zedekiah. When (according to JW teaching) Jesus would come after the 'seven years', he would be the sucessor or Zedekiah, not of Gedaliah.
In their first reply, they explained in detail why the '70 weeks' started at Gedaliah's death. I wrote to them again and explained again that my question was about the 'seven times', not the '70 weeks'.
Then their reply was basically 'The Watchtower always said it like that, even more than hundred years ago'.