I think that most will tell you that helical flechting is the way to go.I have learned this through experience,so take the advice of all those who have been through what you are going through and go helical.

Is there a difference in what is called helical, and what is called straight offset?

I have no problems with any fletch type other than contact issues. I'm shooting a straight 4" vane with thunderheads. I do not try and make fp's hit the same as bh's. To me it's a comprimise, trying to make 2 different things get along.