Why not use BitcoinJ?

+1

They'll lock your account just for receiving too much money...

I wonder if they make more money from fees or from keeping funds.

Good point. If summary disagrees with full history, which one is authoritative?

In current system, transactions only get into block chain once and keep ageing. Old transactions thousands of blocks ago don't need to worry much about current block chain attacks. With summary ledger, they can be messed with at any time.

Current system, only a previous owner of the coin can mess with it. Even under 51% attack, only previous owners of coin can try to double spend... With summary ledger, we take some miner's word for it.

That's a special case if there are 0 miners.

If there were 99% less miners, the same amount of coins would be released.
If there were 100000% more miners, the same amount of coins would be released.

When you mine, you don't increase the supply of coins... any coins you "generate" come at the expense of other miners...

There are a fixed average amount of coins released per hour. Mining is just a competition for who gets them. Think of it as a coin fountain with miners standing around it catching them...