You've had a lot of good suggestions ... and not knowing all the variables of your situation, it's hard for us to know how to zero in on your solution.
I sew in a finished basement with cement floors. I do have carpet around my sewing area ... a nuisance for pins and thread, though warmer underfoot, and some padding when standing at the ironing board. Compromise!
For the heat rises, cold drops issue ... I turn the fan of my furnace on before I go downstairs and that helps to get the air moving, so I get more use of the heat!
If you can, position yourself near where the duct work brings the heat to you. If not, consider some renos to the ductwork to make it work better for you.
And I always keep a fleece vest handy to put on/off as needed!
When it comes to summer ... you'll love the coolness of your basement and will discover it's the BEST way to beat our Ontario heat and humidity! I get a lot of sewing done when it's just too hot to exist ... and as I go up the stairs, the wall of heat hits me!!
