So most of the problems are down to the supplier.
The problem of having two fleets in the same depot is because the new fleet is not yet working - no 745 has yet earned a penny in revenue service desipte the first delivery more than eight months ago.
What you're saying there is typical blame culture stuff. Blame culture doesn't get anything resolved it just breaks down working relationships and means it takes longer to sort a problem and when you've got Greater Anglia saying nothing is their fault when they made a bit on unrealistic assumptions and Stadler saying nothing is their fault and it's the supplier when the same equipment works fine on Hitachi, Siemens and Bombardier products. Everyone is so busy trying to say it's them and not me, that they're not focusing on the problem.
My gripe isn't only with Stadler for not having the units work out of the box, because there were always going to be teething problems, pretty much any rolling stock has unless it's another run of almost identical trains or a subclass from the last one, so GA turning around and blaming Stadler is symptomatic of a blame culture. A good project plan anticipates the potential problems, risk factors and possible delays that could happen and how to deal with them before any train comes anywhere near the UK, rather than just hoping that everything works out fine and blaming bad luck if it doesn't. GA seem to have a planned entry into service date as the same date that they need the units to be in service, which is a very risky.
When a blame game is going on (and this applies to Stadler, especially but also GA) people tend to be less open. Openness is critical with a project like this since it means people will have their say without fear and this promotes discussion and brainstorming, and often helps with solutions. You do that by having the people who made a system discussing it with the people implementing it into their rolling stock and other key stakeholders. Attitudes about 'them' being the problem so they need to come up with a solution aren't the answer here since if people feel that proposing something that involves the other party doing something that will help achieve their goals will be met with resistance then they won't say it and the solution will take longer to find. You need to be solution focused.
Project management isn't just about delivery, it's about solid planning, scoping out potential issues and how you would react to them if they happened, management of relationships between departments and parties and having a culture that allows problems to be overcome, openly debated where everyone feel they can speak up and make a suggestion without fear of being blamed and ensuring that everyone involved is focused on the end goal rather than trying to find someone to blame when there is a setback and stopping people looking out for their own agendas over the team goal.
It's not Greater Anglia's fault that Stadler and Bombardier are running late, but it is their fault that they put together a proposal that didn't have sufficient margin to absorb more of the effects of these things which was always likely to happen and anyone experienced with these kind of projects in practice rather than on paper and in theory would have known that. The problem was that the GA bid team wasn't experienced and it knew the theory of everything and the practice of nothing, the ED knew lots about planes and little about trains and what you're seeing is the results of flaws in the plan that an experienced team wouldn't make.
No doubt those responsible have already left having got their payday for winning the contract rather than being around to see the poor souls in operations who are probably working overtime to avoid cancelling and shortforming more services than the already too large number in cramped conditions. Abellio would need to learn the lessons that First was very honest about learning on the Class 175s in First North Western, when FGE introduced the 360s.