With regards to the point you making about costs. Spreads have come way down in the last 10-20 years due to computers. I mean the whole point is that computers are more efficient than people in some regard. The bandwith HF trader is really not that much compare to the volume they trade. And they pay for the bandwith.

Sure, batches are interupt periods and standard in good exchange markets. You can even trigger them based on volatility measures.

The bitcoin network is absolutely not the same as an exchange. There is a lot of potential in this direction, but in quite different ways. You want transactions to be very, very fast. For example in the first of trading this morning the SPY ETF was traded in cumulative volume of 600M$. That's one product in one hour. Total stock market volume is in the trillions of dollars. You need a lot of speed and efficiency to trade that, and that's what (computer) traders provide. At the point almost all short-term trading is done by computers and there is very good reason for that. Markets work pretty well compare to other institutions (banks, states).