3) you might need the tracking and/ or balancing looking at? also your wheels could be buckled. Mine is pretty solid all the way up to 130ish - done on a private road of course
As for going flat- mine is good up to about 115-120 then begins to slope off up to 130, I think the power drop off is around 5,500 to 6000rpm on a standard car (don't quote me on that)

4) is this only when the car is cold? normal to hear a whirring sort of noise when on full lock and the car cold, should quieten down as the car warms up

not sure about the others mate sorry