Its my understanding that she won't be leaving office until the 7th June although I could be wrong. It would be pretty awkward if she wasn't around for Trump's state visit. Technically the legal default position if a withdrawal agreement isn't agreed in Parliament is still no deal. This is written in law. Unless their's a further Article 50 extension (which isn't guaranteed since it requires permission from the EU 27 member states and France seem to be pretty keen on blocking this from what I hear) or Article 50 is revoked altogether, their's no stopping no deal at the moment. Revoking Article 50 would be political suicide for the Conservatives at the moment though with Farage and the Brexit party already set for a landslide in the EU parliament elections (the results come out on Sunday, by the way).
The thing is regarding no deal, I'm almost certain that the Conservative MP's who are competing in the leadership race now are going to campaign almost universally on the basis of putting no deal back on the table if the Northern Ireland backtop isn't removed. The final two candidates are voted on by the Conservative party membership and make no mistake, the Conservative party members are very Eurosceptic. If it comes down to a Remainer vs a Brexiteer, the Brexiteer will walk it with ease.