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I'm not a pro, but I have spent some time on HVAC-TALK and I am facing some of the same issues you discuss.
Regarding closing off returns... you SHOULD familiarize yourself with TESP measurements (static pressure in ducts) This is easy, but you need to buy, borrow or make a manometer. You also need to verify the TR (temperature rise from return (intake) air and supply duct air temperature. Compare that to the label on the furnace.
Fan speeds... you may be able to lower the fan speed for heat... ( but, not if you don't understand the function/purpose of the above two easy to preform tests.)
Do you know what your fan speed is? For example, a 80,000 btu furnace should have a minimum of 20x25 inch duct.....
What is the size of your return duct... should be equivalent to 16" round for a 80k btu and 1200 cfm.
Are the ducts rigid or flex?
Furnaces usually have a separate speed for heat and a faster speed for A/C....
You may save money hiring a HVAC guy to look over your ducts... the purpose of such is to "prep" your current ducts to a new system you 'might' buy. Since your current furnace is oversized, it's possible ducts will be efficient when you decide to buy a 50k btu unit.
And, it's possible....you might spend lots of money/time and not solve your temperature stratification cheaply...( newer units will have more efficient blowers that operate more cheaply (guess around 30%?) if you want/need to run the fan more often.
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