Biz class seat is the same regardless of 757 or A330
Can post the points to United also, but check the booking class letters and verify you will earn
Flights from LHR typically use the low numbered gates so least walk, from doors opening to being through connections and CBP (assuming not your first US visit) 20-30 minutes should do.
And you will have lounge access at the Aer Lingus lounge in LHR by virtue if your onward from Dublin being in biz