Our tech team thinks the following workaround is possible:
You can write a piece of Javascript code and insert it into WA site which will check its current URL for the string 'widget' and if not found (meaning that user ended up opening WA site in full screen instead of widget mode), automatically redirect to your main site.
In this way user will be redirected from paypal to WA site and then immediately to your site. WA site will still blink for a second but should not be too bad.
I hope this helps. If you need help with this code, let us know.
Dmitry Buterin, Chief Apricot