A true gold standard allows for expansion only so far as the miners can mine it, and the smelters can smelt it. That is to say, not very much expansion. Not a very surmountable problem. So such a standard is attractive only in the sense that it constrains the political ability to degrade the national currency at will, as is happening today.
As to why gold isn't trading at $6500? It isn't ‘worth’ that much, nor is the world economy really ‘worth’ $65T. An ounce of gold is worth one ounce of gold.
I am with you on a pure gold standard. It really isn't feasible in the long run. A bi/tri-metallic might give enough opportunity.
Cornering the market isn't possible, COMEX won't allow it. The system is set up to prevent it, and if it is tried COMEX will change the rules. Ask the Hunt brothers. They almost had the silver market cornered, then COMEX changed delivery and margin rules.
A commodity backed currency might work for a while. But most commodities are consumables, and so shortages of any given one in the basket could skew things badly.
So after all that rambling, I don't have the answers. If I did I'd be rich.
Wisest post of the string.