"Regardless of what happens in our Parliament the final outcome is totally in the hands of the 27 EU member states."
Perhaps only 20 of them.
What will happen can go two ways - if the current deal is rejected by our parliament, the government has 21 days in which to come up with a new one. Potential outcomes are:-
we would exit with no deal
we would re-negotiate
we would have a General Election
we would have a referendum - not on the decision to leave, but on the deal options.
Once a deal has been agreed by parliament an EU exit Bill is introduced. Once that has passed, the European parliament votes on it, and the simple majority vote applies. Then it goes to the EU Council for approval and 20 members have to agree.
Once that happens, it's over - we exit the EU on an agreed date in March 2019.
What happens between now and that date is anyone's guess - we are currently facing a constitutional nightmare. I would not want to be in the situation facing Theresa May right now.