Don’t know much about these. Here is a post from a user whom I was referring to. He mentions a little bit about counter weights on the crank shaft to reduce vibration. But he also mentions what engines worked best for him.
Probably not. As you get more negative collective pitch, it will probably get worse. but I would suggest that you give yourself at least -2 deg for the minimum.
I would suggest increasing your rotor speed to keep within this range. You may be running your rotor speed to slow if you are near 0.8.