I guess you should remake this navigation model with states and events, rather than pages.
Or redefine your pageflow process... i.e. start not from the pageA or pageB, but start from pageZ - from which the actual predefined pageflow starts... and lead user to that pageflow using navigation in pages.xml...
or actually simply don't use pageflow for this path... consider about pageflow as the wizard: i.e.
review basket - >(on some link or button )start pageflow checkout -> provide detailed information on basket again-> provide place for discount entry -> request on payment method -> additional verify -> checkout -> success
What happens before
start pageflow checkoutand what happens after
successis laying in the user behaviour (thus very random).... but the checkout process is linear (actually not linear but with several decisions) with predictable navigation - this is a good place for pageflow.
actually it's useless to bookmark page inside of the pageflow. this pages shouldn't provide such information user like to bookmark. (last is my personal oppinion)