OK, I took a very close look at the whole case, and discovered that the line
IMPR C1 O11 O12 C2
is missing from your topology. Unfortunately, this means that you'll have to redo your dihedral fit. For starters, try 0.44 for your 2-fold and 0.06 for your 3-fold. Also, if you're still suffering asymmetry problems, a possible solution would be to not constrain CCCH while fitting OCCC.
Edit: beware: it is not impossible that CCCH needs to be re-fit as well.
You see, the major asymmetry around the peaks is gone now. There is still a minor asymmetry around the minima, which most probably is caused by the small offset in the location of the minima in the CCCH scan, and which can be resolved by not constraining CCCH while scanning OCCC. I suggest you try that now. Once that problem is resolved, you might want to tweak your amplitudes a bit, although it may not be necessary.
Nice! Next step in the procedure is to take the input file for the water interactions, change "stage" to 5, run it, and see if the water interaction energies are still as good. If not, the charges may be tweaked.
Are there significant differences in the water interaction energies, compared to the constrained case? If yes, you might want to consider tweaking the charges. However, if you decide to do so, you'd have to tweak the dihedrals again after tweaking the charges. Then, you have to calculate the charges again and see if tweaking the dihedrals made any difference. Usually, you won't see big changes at that point (meaning that the force field is self-consistent).