you can find DIR_WS_CATALOG in your includes/configure.php. you will have to change it if you shop is located in a different folder (as it represents th location of your shop on the server)

I am afraid that I don't know anyway of using the images from another server. The problem is that Zen-Cart need to access the images from the serverside (to check the size and the presence of the images) and also from internet via URL.

you could maybe make a script that will update one folder image with another ...
Or you could change of server and get one that allows you to have 2 domains targeting the same folder.

You only need one admin folder in one of your shops.