It may be an accounting trick, but it absolutely expands the money supply when two or more people can claim the same gold. Gold just puts the brakes on government spending, it doesn't solve personal debt that you worry so much about.
If more than one people claim the same piece of gold at the same time, banks go bankrupt, that is what is happening after financial crisis
Not expand continuously, expand ever. If you ever want innovation from someone other than the rich, you need to expand the money supply when the need arises, or there will be no class movement at all. The "continuous growth" of modern economies is primarily a fiction that is encouraged due to the nature of fiat and governments. That doesn't mean that there are not periods where growth is real, and the money supply needs to respond to it.
The growth are all true, but not sustainable. For average people, the biggest spending (and hence the growth for society) comes from housing, then their spending will decrease, unless you could find an even bigger spending after housing, that is the problem now
It is a system that works reasonably well as long money flows, and flows to reasonable areas, something banks are supposed to be good judges of. That is, after all, why we are supposed to be giving them the privilege of holding all of our money and earning interest. It is the banks abusing that privilege that causes the system to shock, not the fact that debt exists. And it is the politicians that are so easily bribed into doing things that are favorable to the banks. This again is not an issue of debt, it is an issue of corruption of the monetary system. The fact that the system is corruptible is a big problem, but what do you expect when you give that sort of control to a few people?
Yes, those aspects all affect efficiency, but my model just showed that no matter how smart/efficient the banks/policy maker are, that debt based money creation will always lead to insolvency
In my second model desribed in the OP, you can try all different productivity/pricing combination to generate saving for both A,B and D, you will find out that is simply impossible, their combined net worth in currency's form is always 0, just because none of them have the original ownership of the currency