The EU needs Brexit to fail, but it can’t be perceived as needing it. It will accept the UK back in a heartbeat while leveraging fear and propoganda stirred up by the likes of Tony Blair.
All May needs to do is send an Art 49 letter to Brussels undoing the Brexit process. No further action is needed to stay in.
The worst thing for the EU would be a hard exit that works OK. The EU would fall apart in a couple years if that happens. The EU is counting on May & Pals cancelling Brexit, which seems the most likely outcome now.