That's good for the UK as they can test the waters and revoke the notification if they don't get the deal they want only to activate it again and start from scratch. Its the sort of thing that would destabilise the EU which is something most Brexiters want
Not really, they can't revoke the notification, I suspect that Lord Kerr believes that the EU will diplomatically pretend that nothing happened if the UK wanted to stay but that's not a given.