Are all heat-exchangers of 90+ high efficient furnaces made of stainless steel?

can any qualified hvac out there show me a computation on how the returning air is affected by moving the A-coil on the RA . My hvac guy is arguing that I dont need any extra return opening above the area where the A-coil was moved to. Doesn't the presence of the A-coil in the RA cause an obstruction to the air flow inside the RA.

If the ductwork was set up properly to begin with, as long as the coil can be used for either "push through" or "draw through" operation then it may be ok (if the rated static pressure values of the coil are close).

Duct work is designed by an external static pressure (ESP) calculation. All the fittings, turns, sizes, etc are calculated in equivalent feet measurements. The ductwork is sized according to this calculation.

Have your technician show you his calculations to prove it will be ok.

One other thing to remember is that the condensate drain will need a p-trap installed to allow proper drainage if the coil is in the return air stream.