Quick Sit Rep:
1) May's deal is dead - even if Bercow allows it back, the DUP will kill it.
2) There is no majority in the HoC for any other proposal.
3) If the deal is not passed, HMG has until 12 April to tell EU what the plan is.
At this point, there are only 2 options:
1) Withdraw Article 50; or
2) Leave on 12 April on WTO terms but without any deal with the EU.
Withdrawing A50 will lead to chaos - not just 17.4m unhappy campers but there are 30+ Tory MPs who will thereafter rebel on everything and will support a 'no confidence' vote against the govt. Party discipline in the HoC will collapse. A general election is inevitable but that will not fix the split in both main parties. Even after an election, gridlock is guaranteed.
Leaving on WTO terms will provoke an instant response but when the dust settles, a GE can be called and people can have their say. Once the UK is out, everyone can rally round to decide what to do next. Some will want to rejoin the EU others will look to trade deals as the way forward. But that is a normal situation and can be debated.
The least long term damage to the system itself, will be to leave with no deal.