1) If they passenger puts a block of lead the size of 55x40x20cm in the overhead and it falls on the aisle seat passenger snapping his/her head when the bin gives, LH has a problem, not the passenger who put the luggage up there. Hence the weight restriction.
2) Usually Y pax store their stuff in Y. I'm yet to see the usual practice common in the US that people (especially elite boarding early) dump their stuff in the bin in front and then move back
3) C gets two pieces, see post#1
4) agree