Hi There.
Easyjet use a free seating policy because of the way 'us brits' work i.e we dont like being bunched up together, so we'll go where we'll get as much space as possible, usually as far away from others, so unknowlingly to them they'll distribute evenly throughout the cabin.
Weight and balance with easyjet is always done by a loadcontroller who will be passed information from the outbound crew, or if the crew were already on board the aircraft they'll pass that to the dispatcher who'll relay the information to the load controller such as the block fuel, burn, taxi, crew, DOW, DOI and any performance restrictions such as MTOW's etc.
Baggage is always placed into aft hold 4 (up to 3469kgs)on easyjets 733's, then any excess goes into forward hold 1.
Also, with regards to the headcount, i always get a headcount from the number 1 cabin crew before the doors closed, as a few weeks ago a whole flight had to get their boarding cards out to be checked because one of the gate staff had allowed someone through with a boarding card, so they thought. Turned out to be a calculation error with the gate staff as the number 1 had the correct number, but the gate staff didnt.
Hope this helps