If we pass the WA then A50 will be extended to allow time for it be properly ratified and implemented on both sides.
The ratification is not automatic. The WA has to be approved by member states under QMV and pass the European Parliament. As Barnier has acted within his mandate and their is consensus on the EU side this is likely to be straightforward.
The WA passing the house is ratifying the agreement but it does not implement the WA. That is done through the WA Impementation bill which along with the Trade Bill, Immigraton bill and a raft of other legislation replaces or converts EU law into U.K. law and allows us to set tariffs etc. Absent this legislation exiting becomes a legal, regulatory and security nightmare which is why you have the warnings against absenting without a deal.
The Govts no deal solution is to rely on the goodwill of the EU and our neighbours to help us out of this mess with temporary interim arrangements which will come at a price and are unsustainable in the long run. We are creating this ‘goodwill’ by threatening them with chaos. Go figure.