They've sent all the old trains away and are denying it,
GA have been horribly optimistic when it comes to the introduction of the new fleet. Their plans were based on the assumption that the trains would be turn-key and ready to go, despite involving a manufacturer who has never built anything for the National Rail network before.
This is interesting on its own - parent company Nederlandse Spoorwegen have burnt themselves quite badly in this way in the past, twice, but have learnt some lessons from it. Whenever NS introduce a new fleet they keep the existing trains on standby.
It looks like none of those lessons were applied to GA.
they've ordered trains that we are all now learning are not really up to the job,
They are facing some technical issues, which need resolving. This on its own isn't uncommon for new trains - as GA rightly indicate, each fleet is built tailormade for the country/area where it will run. Unlike cars, trains never "work out of the box".
I have heard rumours that they have now resorted to removing the pantograph on the new trains.
I highly doubt this. They ordered trains with pantographs for a reason - it keeps the costs down (electric is cheaper than diesel) and it also lets the trains have smaller fuel tanks as they'll run electric for part of the route.
Recently a pantograph failed to drop and it subsequently hit a bridge. Whilst this is being investigated, the trains are operated in diesel only mode.
Why can't they do what they use to do in a shortage and hire trains in from other operators, such as DRS,
Those trains need to be available in the first place and even if they are, crews need to be trained on them. That's a logistically rather complex operation and not something which is done on a whim.
and more so, why didn't they just order the right stock from the beginning, such as mk5 coaches and caf DMU's or something like that?
It's hard to say what "the right stock" is! The GA bid team decided that the Stadlers and the Bombardier Aventras would be the right stock for their future franchise. Things aren't going exactly to plan though.
For the Stadlers, one could say this is down to inexperience. GA were taking a risk and, if they had planned wise, would have put all the contingencies in place to allow their old fleet to continue while the Stadlers bedded in.
For the Aventras, from a reputable builder which has plenty of experience building for the UK, it's just a painful story. The 720s should've been in GA-land for a while but they are nowhere to be seen...