Your knowledge is limited, hence why no one replied.
Indeed, P=V*I and V=R*I. Resistance on a TEC is not linear with amperage/voltage, but let's assume it is.
Yes, if you undervolt a TEC, from the max of 15.4 down to 5 volts, it will operate at a lower power level.
You can use two PSUs together, to generate 10 volts, as long as you know what you're doing; watch out for ground faults, and make sure that you don't exceed the amperage on either PSU. There's an article on this site about it.