i was able to get it to work in sandbox mode. add item to cart, checkout, get directed to wepay, enter CC info, complete order. i even had a customer (who i now have to call) put an order through when i had it in testing mode.
i didn't even download the "dumbed down version" that you posted. i'm using the regluar OSC plugin you posted initially. i'll PM you a link to my site.