[LazyPup]

Either your plumber just doesn't want to be bothered with the problem or his/her knowledge of the toilet bowl function is seriously lacking. In either case, I would consider consulting another plumber before authorizing any work.

I am attaching an illustration which I prepared some months ago that shows a cross sectional cutaway view of a toilet bowl.

As you can see in the illustration there is no internal passage from the drain trapway to the tank to bowl connection point. In fact, when a toilet flushes the water in the bowl rises quickly and as the water in the bowl rises it also rises at the same rate in the trapway passage. When the water reaches the top of the trapway and begins falling down the the descending portion of the trapway it creates a syphon action that sucks the remaining water out of the bowl up and over the top of trap weir. If there were a passage from the top of the trapway to the tank to bowl connection that would allow air to enter the trapway and would instantly stop the syphon action, which would then prevent the flushing action.

in order for sewer gas to leak from the tank to bowl gasket it would first need rise up from the drain, through the trapway and through the water seal at the bottom of the bowl. It would then need to rise up through the water inlet holes in the upper part of the bowl, pass through the internal water passage, then leak out through the gasket. This would be a physical impossibility because even if the sewer gasses were to bubble up through the water seal at the bottom of the bowl, once they enter the bowl they would be at atmospheric pressure. Now considering that sewer gasses are heavier than air they would either remain in the bowl or in a worst case they may be forced out through the air gaps through the toilet seat. Under no circumstances would there be sufficient pressure to force the sewer gasses to enter the tiny air inlet holes in the water supply ring, but for the sake of argument let us assume they did get into the water supply ring. The gasses would still be at atmospheric pressure, consequently there would be no pressure to force the sewer gasses to leak past the tank to bowl donut gasket.

As you stated in your original post, the wax ring at the bottom of the trapway creates an airtight & watertight seal between the top of the closet flange on the drain line and the underside of the toilet bowl. While a defective wax ring is not the only possible cause of your problem it certainly would be the most common cause.

Changing a wax ring is a fairly simple and inexpensive (typically $1 to $3) DIY repair and if you would like to attempt it I would be glad to post a complete step by step procedure to help you.