Can you share some screenshots? I might be able to help?
Though it definitely sounds like one is through Paypal processing and that might have different settings than through the standard shopify checkout processing.
I've looked around the web, and tried to play around with the settings myself.
But it looks like you are right, I can't find a way to change the text for that button on return.
Maybe someone from shopify can help you, but my bet are the chances are pretty slim.