With all due respect, everything you have written may or may not be true, but with absolutely no knowledge of what is in any of the contracts, nobody knows. The only thing we do know is that the EU are at a disadvantage having signed the contract later but, as I pointed out above, my experience of contract law (in very different fields) is that makes very little difference to the detail of the contract
As I said earlier, I suspect more will come to light over the next few weeks and hopefully the AstraZeneca/EU dispute will not require a re-examination of the UK/AstraZeneca contract
There must have been so many caveats in all the contracts regarding this. When signed no one was sure the vaccine worked or was safe. The EU still haven't passed it. There has to have been many grey areas.