I had a similar draft/smoke rollout problem last year with my handfed homemade stove. I too had a 6" chimney a horizontal section, and a similar height chimney on the outside of my house. Same setup as you, only I have SS insulated chimney, and a basement stove. I did lots of tinkering, from hinged smoke flaps on the load door, single-wall extensions to the chimney, 45 degree elbows to replace the 90, etc.
I never did solve the problem, as I installed a stoker this year instead. However, I believe what I really needed to do was get an 8" chimney, at the very least. There's just too much fuel being burned, creating too many gases to get out a 6" hole.
Prior to the handfed stove, I had a woodstove. It was borderline - as long as I had a clean chimney and a hot fire, I could avoid smoke. But with the coal stove, smoke rolled out even with a clean, hot chimney. It's very frustrating.
Don't get down on the Harman stove. I think it's a really good stove, and I know there are a few people using them in Wyoming that have good success.
I'd take Berlin's advice and upgrade the chimney. Problem is, it's not cheap, as you know.
Steinke in Wyoming