Check the value of TR4 (5k, marking 502) and R32 (2k7, red, violet, black, brown, brown).
What is the max negative value you can get on TP6 (trimmer fully CCW) ?
Do I need to adjust the output potentiometer back to zero (instructions does not mention to do this)?
No, the outpout pot is set to get 6.0VAC on the XLR with the jumper removed.
Also do I need to pick a sidechain setup step to go with(i.e. -3db @ -2v, or -6db @ -3.6v)?
You need both to be set.