Terrible mail experience with upgrade

Upgrading to OS 10.11 has been a disaster for me, the worst par of which is that it did not import my saved mail and mailboxes as well as about 150 mail signatures that were created over about 10 years. This is a catastrophic situation which 3 different senior Apple advisors have been so far unable to help me with despite perhaps 6-8 hours of working on it. I have all the previous mail data from a Time Machine back up, but don't know how to get it into the new Mail app. Help appreciated.