The turn signal light coming on is feed back from the controller output. Somehow the brake output is connected to the left turn signal. Try disconnecting the trailer, see if the problem goes away, and double check the connections in the truck plug. If that fixes it, there is a problem in the trailer wiring.
The knob underneath is the gain. You adjust that to set the sensitivity of the trailer brakes.
Usually a good method is to make a series of stops from about 20 MPH on level, dry pavement, set it so the brakes don't lock, and you get a good feel for them. That will need to be adjusted for each trailer and depending on the load.
As far as the angle, if it is the "swinging pendulum" type, yes it will make a difference. Once the wiring is right, and you play with the gain, try it, see if it works. If not, try leveling it out, reset the gain, give it another try.