On 3, I think it depends on your time-travel rules. Because the first thing that would happen is that people would flock to retool to produce the higher value commodities (if corn price > wheat price, people would stop making wheat and start making corn), causing a supply glut and price collapse.
I think based on "invent a time machine" the implication is that Perfect Trader X (I imagine him dressed like Racer X) would make a trade, then gather future information based on his latest trade, then make the next trade.
