Hi Rick,
I had/have the same issue. In Dovecot 1.x I was able to use LDA with vpopmail and only environment variables to deliver to a home directory. I was never able to get it to work with Dovecot 2.x - don't know if using environment variables is depricated, but I ended up having to setup an auth server to use lda with qmail/vpopmail. :(
My hosting server uses vpopmail and lda, and I don't recall having to do that. If you want to compare configs, let me know.
In this case, I authenticate against samba active directory, and a (seemingly) small config change there led me on an epic quest of configuration updates through the whole email system until I got stuck in this lda box that didn't even have a problem in it. sigh. I was doing so well....
But you showed me the outside of the box:
For your environment variable questions, those come from Qmail. See: http://www.lifewithqmail.org/lwq.html#environment-variables
It's stunningly obvious now that you point it out. Soooo obvious, in fact, I probably would have been weeks figuring it out. Thank you so much...
change my defaultdelivery to:
|HOME=/home/mail/$LOCAL /var/qmail/bin/preline -f /usr/local/libexec/dovecot/dovecot-lda
and lda looks for the correct directory again.
I guess environment variables are not deprecated, presumably that's good news?