With regards to "mind chatter", I'd suggest looking for a cause outside your meditation. If work has me riled, I have a tough time getting still. When this happens, I remember my bicycle racing days when a buddy of mine used to enjoy difficult challenging headwind training rides because of the extra "oomph" they gave his racing. When I'm challenged by work or home or whatever, I just think of training in a strong headwind.
As for chair or cushion, my legs/knees are not sufficiently compliant for a cushion and I agree that a chair is challenging in its own ways. I have found that a seiza bench is both very comfortable for me and extremely conducive to the work at hand.
The birds have vanished down the sky. Now the last cloud drains away.
We sit together, the mountain and me, until only the mountain remains. Li Bai