If the Withdrawal Agreement cannot be set on oneside so as to agree a new "way forward" then there can only be the OLD "way forward" - making it May's deal or no deal!
I see no other way of reading the words in para 4.
The only thing that can change is the political declaration but the back stop and the whole of the Withdrawal Agreement will remain whatever the Commons agrees! ( Just listening to an FT journalist saying that very thing on Radio 4 PM.)
Seems like the HoC needs to tell the EU what is wanted by the majority of MPs and if this is not the Withdrawal Agreement then this should be shelved - if it is not then we should leave without a deal.