The statements about batch processing are interesting. For example, when you transfer money to someone at a different bank, one way is for the banks to have correspondent accounts with each other, so the transfer is just within accounts at one bank. When one of the correspondent accounts is out of money, they have to do a real inter-bank transfer, probably via the fed or some larger bank.
Bottom line is that there is parked money in these accounts. The banks should have every incentive in the world to minimize this parked money (either to get access to their own money or to avoid paying interest on someone else's). So there should be pressure to make the systems as real-time as possible, not batched.
Banks already do some of that locally and internationally.
In South Africa we have BankServ, which is a local interchange and clearing house. Transactions are still in batches, but the settlement between banks happens nett between the local banks.
Internationally, MasterCard, VISA, Amex etc. manage the clearing. Banks then have to pay within 2 days via Swift to either other banks or their designated international settling bank.
Going back to your suggestion, it could work because the banks could just pay each other when they run out of funds. The challenge becomes fraud and other legal issues, which often mandate settling houses.
The real-time problem is a legacy hardware problem as well as the settling issue.
On your last point, money is normally parked at the Central Banks, mostly as a legal requirement. From my experience, banks in healthy financial economies like mine don't keep a lot more than the minimum cash reserve with the Central Bank.
Yes. All current "direct transfer" solutions, more or less requires that you keep a rather large sum of money in a common pool (at the 3rd party) to cover for the event that you run out of money and have to close the bank, to guarantee that all (or at least most of) the money is actually available when the debts are settled, typically a few times a day.
I don't think real time settlements involving the central banks will happen any time soon, but maybe more frequent batches.
Bottom line is that there is parked money in these accounts. The banks should have every incentive in the world to minimize this parked money (either to get access to their own money or to avoid paying interest on someone else's). So there should be pressure to make the systems as real-time as possible, not batched.