How much is partial? I take it that there are other air returns in the system which are not blocked. It might unbalance the air flow but you won't know unless you try.

I frequently block about 1/2 of the main floor return to improve the upstairs cooling and about 1/2 of the upstairs return to improve the downstairs heating.

YMMV

As far as I know, it's not a good idea. Besides reducing the designed airflow that will alter performance, whether through the heat exchanger or coil, depending on the season, it also puts added strain on the blower motor.

When in doubt, check with your HVAC contractor.

Are there other returns beside this one? A 6" x 20" return by itself would be too small for the smallest furnace or AC. If there are additional returns then partially blocking this one may be alright.

Will the back of the love seat be placed against the wall? If it is then see if you can pin the back skirt so it stays up and keep the love seat 6 inches away from the wall in order to allow air flow.

Great suggestion about pinning up the back portion of the skirt. Yes, the loveseat is against a wall and bringing it in from the wall should not be a problem. This is the only return in a 13' x 14'office which is housed in a recently renovated and updated Victorian house, with new heat and a/c. The office door, I am told, will be open most of the time as well. Thanks so much, we'll give it a try.