Right, so leave the courier-imap package as is. I've only been using ispc for a few months too but I've been using the full courier-mta suite for about 10 years on a couple of client servers so I am not used to postfix at all. Yes, I can see the dot-lock path is wrong although I have had email delivered to my inbox... until maybe I rebooted, not sure. I'd like to try and understand why this error is happening rather than patch it up because I want to add Dspam filtering and maybe even become bold enough to try courier-mta instead of postfix on a test server... but to do that I need to understand the system better which is why I switched to courier-imap because it's half way to using courier-mta.
As for a maildroprc there isn't one unless your create it but it's almost useless without courier-mta providing a bunch of env variables that maildrop needs when using a global /etc/courier/maildroprc. A maildrop filter filter in the /var/vmail/domain/users virtual home directory will probably work but I haven't got that far yet (not sure what the magic filename is yet). I use this to test with and on a standard courier-mta system those env vars are available but as you would see they are not when postfix is the MTA...
~ head /etc/courier/maildroprc
log "$RECIPIENT -> $HOME"
Obviously touch /var/log/maildrop.log; chmod 666 /var/log/maildrop.log