I am not sure there's actually a shortage of drivers, rather issues with their training with some not able to sign 700s and others not having the route knowledge.
700 drivers need to go to WGC to take out the morning trains as no WGC drivers sign 700s yet (they'll do this with the 717s and also learn the routes through the core too), but recently many don't arrive in time. Once the later trains return and go into the sidings to come out, they are pretty much a given as running, but in the morning there can be 700s in the yard that can't come out.. and we all know how the current system likes to show them as 'on time' before they disappear (the only fix is to have them manually marked as cancelled, as they have no inbound working to allow the software to work out they can't run).
Many Moorgate inner services rely on having a relief at the other end. Trains cannot be left at Moorgate, so Finsbury Park is where they switch. But if a driver can't get to FPK because they're stuck at King's Cross then that's cancelled or delayed heavily, maybe having to skip stops or even go to Hornsey (or WGC sidings) to get out of the way.
As many drivers are based at WGC but originally from Hitchin and surrounds, they have to be able to get home.. and then of course they have to work within their hours.
At times there are lots of drivers spare doing very little during the chaos because they're not allocated to a train or have been asked at short notice to drive something else, but they then are useless later on as they've reached their hours and so the fun starts again. They can take a train to somewhere, but not back, so there needs to be a relief. And where are they? Who knows... Train gets cancelled even with a driver ready to go.
King's Cross have similar problems, with the added issue of what happens with many hundreds of people on the concourse that board a train (a 12 car 700 holding 1700 people) that gets cancelled.
All of this is why each driver needs a proper diagram and that is what went wrong, and should now be fixed. I am not sure the rosters changed as I know that was going to be fought hard, but at the end of the day, they should have enough drivers. And where the training has fallen short, using some 365s may bridge the gaps. Off peak, you can allow for less drivers being available because you're running fewer trains and have some doing the route knowledge to allow more trains to be run.
That's the theory. Let's see the reality.