the only way to balance the tempature on a two story house with one unit is to zone the system. it dont matter how few or how many returns you have.zoning is the way to go. dont waste your money trying to do this and that because it won't work.

I would try blocking off the return on the first floor, this may draw the conditioned or heated air up to the second floor.

Yes, the heat rises, so the heat rises to the 2nd floor.

When I put a system like this in a new house I always put the return as high up as possible on the second floor.

Can you block off the stairway ? this will keep the hot air from rising and being trapped on the top floor.

Also, I would damper down the first floor registers and make sure the second floor registers are wide open.

To run the fan continuously, you may be able to change the fan speed to the lowest setting where it will be very quiet.
Read the manual.
As far as zoning goes, it may not be possible with your model furnace and it will cost some good money, so try this first.

As someone in the business for 20 years and growing up in a HVAC family I would say your idea to consult a professional is the right thing to do and an estimate on new equipment should be free. A few things I have noticed. First your heater is probably too small to move enough air for the size of your house. This can affect the AC as well. Undersized systems usually affect the place farthest from the indoor equipment. Second the noise is not because of high efficiency. To the contrary it may indicate a problem with the blower system. Third never block air ducts or return air ducts as this reduces the efficiency of your unit and can cause damage. If you are adding onto your house you will need another or a larger system so perhaps adding a second system would be the best bet. My advice get a professional. The money you save in energy costs will be worth it in the long run. Good luck with it.

Here is a link that might be useful: Ra-Jac Air Conditioners and Heaters Texas City TX