How can I tell what is causing commerce to throw entity wrapper exception errors?
sounds to me like the rules the old developer put in place are still firing for coupons and since you tried to downgrade from 2.x to 1.x they're firing and its killing your page ........ look for rules that might be lingering about??

More debugging can be found by installing devel, finding this path in the sourcecode of commerce_coupon /admin/structure/views/view/commerce_coupon_review_pane/preview/checkout/ajax and using dd() on func_get_args() throughout that pages' page callback function to debug where php is trying to get currency_code.

In general installing and hitting the page with xdebug debugger, and stepping through the code, could be very helpful too -- learn how to set some breakpoints in the debugger.