As far as I know rossignol doesnt use a mountng plate in they're park skis.
A few cm's forward effects the sidecut alot more than the flex. The change in flex is negligible
It does not void warranty. Rossignol just drew 3 random lines on there anyways.
the addicted line is not the center of the ski. I believe it is -4cm back from center
yes the blast are a cheap kids ski but its decent for a lightweight rider. I've been riding the adult versions for years and had no problems out of the ordinary.