I don't understand why some people find it so difficult to understand how stadiums are financed. Just think of buying a house with a mortgage, it is really quite simple.
The key point you are missing is that we exist in Scottish football. You maybe missed it but banks are no longer lending to Scottish football clubs as there have been too many fingers burnt in the past (Motherwell, rangers, Dundee, Hearts etc etc including our good selves where the Bank of Scotland wrote off £10M in a non recoverable debt). I guarantee that we won't get a bank loan of more than about £5M out of the £50M we need (most likely secured on future naming rights income stream to protect the bank).
You compare it to a mortgage on a house but it is utterly different. The bank lends you maybe 75-80% of the value of the house as they know that there is a very high probability they will get their money back if you default on the mortgage and they have to sell your house. If the bank lends us say £25M of the £50M we need and we default on our loan how exactly do you think they will get their money back? Who are they going to sell a second hand football stadium too? The only answer is whatever reincarnation of AFC then comes after the current AFC and they will offer pennies in the pound to buy the stadium as they will be the only buyer (remember the land has no planning consent for housing nor is it likely to get any so knocking down the stadium and selling off the land for housing won't work for the bank).
The stadium will only be built if current and new shareholders put up something like £25m-£30M of new cash from a share issue. Who knows if that will happen, probably only Dave Cormack and friends.