Is there a mail app handling maildir format on one's mac in a reliable manner?
I can only suggest that you run a local mailserver on your Mac using Parallels and on there you create a virtual machine being Ubuntu Linux. Strip down the install so you have Dovecot, Postfix and Getmail, and of course the GUI bits. you can then instruct Thunderbord to read mail from  Maildir store. I had this done and it works perfectly. One caveat though is that the VM MUST be able to connect to the client. otherwise you end up with a mailstore that is not accessible by Thunderbird, nor can it retrieve Internet email. YMMV depending on Linux knowledge.