I don't know. You could always test it. My guess is that the $ThankYouPage URL will take precedence over anything you set in PayPal. Of course you could set them both to be the same thing to take any guesswork out of it.
Currently I do not have the PayPal "Auto Return" feature enabled. If I do enable it, will this interfere with LimeLM in any way?
Also, in your HTML form code for PayPal, there is an input field called "return" which gets populated with the $ThankYouPage URL from PaymentSettings.php. If Auto Return is enabled, will the $ThankYouPage URL still be used? Or is it overriden by the default PayPal URL as specified on their site?
I don't know. You could always test it. My guess is that the $ThankYouPage URL will take precedence over anything you set in PayPal. Of course you could set them both to be the same thing to take any guesswork out of it.