TBF, it doesn't NEED to.
If he can form a GNU in the first place then IDEALLY he'd pass a law marking a No Deal Brexit as illegal (of course, any future government would rescind that law, but they'd have to go through due process).
IDEALLY, he'd set in process a second referendum, followed by a general election.
He'd PROBABLY request an extension to article 50 - which would take about 30 minutes, we can get on with other things whilst waiting for a reply.
The only thing he'd NEED to be in power long enough to do, is to call a general election, with himself as PM and his cabinet in position to follow process whilst parliament is closed; but most importantly, with Corbyn setting the date of that GE.
That's why Ken Clarke is the best choice (Harmann second) as they wouldn't "undermine" Corbyn's position, and are much more palatable to pretty much every MP who isn't in labour (and several Labour MPs as well).
Corbyn would probably get enough support to request an extension, and call a GE; Clarke would probably get enough to do the rest.