I'm with you OP, I think it's an utterly stupid idea, and is only happening because this area of policy has been taken over by nutters and zealots -
I have met several of them. Like any other extremist, if you try and ask questions or raise any doubts you are shouted down as being a denier.
One of the leading lights of this movement is an arrogant obsessive who clearly thinks that Britain was a much better place in the Middle Ages, when there were far fewer people and more animals.
Where they have been reintroduced already (Germany and Poland) they do millions of Euros worth of damage every year, and in Tierro del Fuego they caused terrible ecological devastation.
There are millions more people here now, with far greater levels of urbanisation and infrastructure than when they were last a feature, around 300 years ago, so the balance of nature is now a completely different ballgame. It is stupidly naive to think that they will just slot in unnoticed.
One beaver fan has already lost his pair because they don't like the place he wants them to settle in, you cannot control them once they have been released, and OP points out they will breed. This means they will either have to be culled or cause huge amounts of damage. In Scotland they have already damaged flood protection measures that had been put in place. They could potentially destroy precious habitats like chalk streams.
I despair at the folly, and all the bunny huggers believing that it's going to be lovely.