Setting Default Permissions

Ok, for point 1, the solution is quite easy:

chgrp developers /opt/dev

For points 2 and 3, I suppose you'll need ACL. So, the first thing to do is to edit /etc/fstab to give the option acl to the mountpoint of /opt/dev. If /opt/dev is not on a separate partition you'll need to enable ACL for the whole root filesystem.

Then you'll have to follow this answer.

chmod g+s /opt/dev

should be equivalent to chgrp developers /opt/dev (and doesn't set the sticky bit, s sets the suid bit).

Then proceed with setfacl:

setfacl -d -m g::rwx /opt/dev  //set group to rwx default 
setfacl -d -m o::rx /opt/dev   //set other

to set advanced permission criteria for files and directories. To be honest, I couldn't find a way to set separate file and directory permissions, but I'm pretty sure it's doable. You can try this tutorial to have more information about the topic.