Fire of all, this is no longer an argument, it is a lecture. I am educating you. Feel free to take notes.
Secondly, the whether the temple priests could use pagan coinage for personal purposes is academic, since that never be and an issue in practice. Most likely the priests wouldn't have so much as touched graven images, but that is neither here nor there. The temple priests were collecting a temple tax, and it had to be in inoffensive currency. Collecting graven images as profit in a currency exchange would have been at odds with this.
The money changers were essential licensed professionals. Their function was critical precisely because the temple could not traffic in the pagan money that was so prevelant. You mistake is based on an accretion of misconceptions that are pervasive and based on medieval anti-Semitic stereotypes that the money changers were in league with the priest a to gouge the peasants. This is false, as we now understand through modern scholarship.
In fact, the money changes were lauded in their time, and a common Jewish saying of the time encouraged people to "act like a money changer," which is to say to conduct heir affairs honestly.
The money changers were honest, third part businessmen who were plying their trade in the temple. That is the problem, that they were profit seeking in the temple, which, as Jesus instructed us, is the wrong place to conduct business, even if it is honest. Contrary to popular myth, the money changers were not gouging, they are providing a valuable service. Contrar to your argument, they are not fund raising for the temple, either. It was a personal profit-seeking business.