I have never used the EZ-Flex suspension but I have thought about it; it looks like it could help. But I keep thinking about how many trailers are made without it and they all seem to work; is the cost worth the improvement. What is the old saying “if its not broke don’t fix it” in your case maybe it needs fixing.
To the axle issue, I have never seen or heard of having axles that are two different dimensions. I would have to believe that if there was an application where this situation could be better we would see it used. But, you do see double even triple, dually axle trailers, maybe this just confuses the point. Even a double axle has more stress on suspension than a single axle. So this leads me to believe the first shop was wrong to install a mismatched axle; the trailer suspension is designed to handle specific axle set ups.
There is an easy-ish measurement to see if your axles are aligned: mark the furthest most center point on the tongue( center of where the ball meets is the key) of the trailer and then measure to the axles bottom bolt to the backing plate of the brakes.
With all the axle/ suspention issues I hope you found a really good trailer shop, not necessarily a camper shop.
Good luck, wish I could help more