As for why your PP Express Checkout is failing, take a look a this thread:
showthread.php?t=11083
And see if it describes what is happening to you.
As for the fix for the checkout page being wrong, Eric Mittman's description of what to do a few posts back is accurate. That is:
Update the <a href="checkout.php"> link on the pp_cancel.php [and the pp_checkout_failure.php] page[s] and point [them] to cart.php instead 


