Important note. The situation with the swash plate. Make sure the main drive gear is seated right. Hold the Heli and gently lift the main rotor. If it is moving up and down, you'll need to slide the white gear up the shaft to until there is little to no play.
Cannot explain the odd trim issue. May have been the main shaft issue. It will be better balanced at the full rates. like I said before you loose precession in low rate mode. If you had a dx6 or better tx, you could a dust the rate as well as the throw of your inputs. I.e. You could give it full range but slow down how fast the servos move which would make it handle easier, but not as easy to get crazy. Lol.
Switching to the longer pins will really make it nimble. In both rate modes. I haven't change mine yet, but would guess it would improve the low rate mode controls without going crazy.
I like to use full rates for spot landing and low rates for flying. At least in the house. In a gym i let her rip.