Is my plan for moving return duct a few inches ok?
Here is what I would do. First, move the condensate pump to the left side of the furnace to get it out of the way of the return duct. Make sure that you add enough pipe unions so the pipe could be removed for furnace service. You may even be able to use a flexible tubing instead of the rigid piping. I would then measure from the bottom of the return duct to the wall to see how far I could move the duct. I would draw a diagram of the duct I would need with an "S" offset to move the duct to the right as per the drawing. Disconnect the whole return drop from the horizontal piece to the furnace connection. Take the duct that you removed and your drawing to a sheet metal shop and have them fabricate the "S" offset or a completely new duct including a filter rack. Also have them add a canvas connector to the vertical run to make installation easier. You could then take the new duct to your home, install it, and you now have an easy filter change and a correct duct. If this out of your expertise, you could call an HVAC shop to do the work. Hope this helps