Coming in late on the A/C problem:
I would think that turning the temperature control clockwise would raise the temperature threshold for when the compressor kicks in and begins cooling.
I'd try just barely turning the temperature selector clockwise just past upright to see if that is the case, rather than all the way clockwise.
As for the exhaust pressure, my 10k BTU portable has a really strong squirrel cage fan built into it to push out the hot air through a 5" flex hose, and it really pushes the air.
Compared to my 265cfm squirrel cage fan that I use to exhaust the hot air from my light hood, the fan in the A/C unit MUST be at least 400cfm, because air shoots out there's like no tomorrow.
The other thing is if you're venting out into a chimney, the air mass in the chimney might be too much for the A/C's exhaust fan to handle, since it's meant to vent directly out of a window or wall.
It might not have enough oomph for that and would cause back pressure in your A/C system, and make it not perform very well.