I cannot imagine any scenario that would result in Cable being Prime Minister.
The Electorate quickly change their minds. In 2015 they gave the Conservatives a good working majority, and two years later, the Electorate didn't give them any majority at all. Public opinion had shifted. A year ago, the electorate narrowly voted to leave the EU, but that does not mean they want that this year, or in 2019.
The Lib Dems are hoping that the public have lost their appetite for brexit, and they may not be wrong.