Put the screwdriver away. You're going to make it worse.
Find some good vice grips.
Now find some all thread that is the same pitch as the thread of the vice grips.
You can now kludge together a vice grip slide hammer, similar to this
https://www.eastwood.com/eastwood-lo...xoCZyAQAvD_BwE
Soak the seatpost in some penetrating oil. This should be for a while day or more. This will give it time to work and also time to step back and breathe.
On Sunday, at the earliest, attach your vice grips slide hammer and get to work. I'd start with short gentle taps, like 50 of them, increase force only if it doesn't work.
Also, I'd put the end of the tool in the vise, and move the frame. It would give you a much easier way to secure it all without clamping the frame.