The simple rule of thumb is 429 MP's have to vote for an early election, or there is a vote of no confidence in the existing government and a vote of no confidence in the next government. The first is impossible (as it means Labour voting for an early election) the second is more than possible. May triggers a no confidence motion and whips her MP's to abstain, stating that she wishes to have a mandate (similar to Schroder and Merkel a few years back) and so the motion is carried and the government resigns, Corbyn is then asked to form a government and 14 days later presents it to the House, the Conservatives (who have the majority) vote against it and we have a general election.
That's something I've always wondered about the new fixed elections law in the UK. Your scenario is more convoluted than I think would ever happen. If the PM is trying to force an early election, it's likely the opposition doesn't want it (like right now) and wouldn't have any part of the Government's no-confidence motion. As bad as the optics may be, I think it's more likely the Government gets defeated by its own majority in Parliament. Like I said, it may not be a good political move in the immediate term, but ultimately, people really don't vote on issues of process. How often do governments actually lose a snap election
because they called a snap election? If May were to force an election by the end of the year, she's almost guaranteed to catch Labour at its worst.