Lounge - yes, one guest. Same PNR is NOT required. In fact, the passenger doesn't even have to be going to the same place or necessarily at the same time although the person does have to with you at point of being guested. I've guested someone in who was flying to MAN much later when my EDI was on final call!

Luggage - not sure. I am fairly sure the privilege carries through on a BA booking although to be honest I have never pushed the boundaries as BA are generous anyway . On AA I would be surprised if the same was true.

Seating - on BA you can use your Silver/Gold status to assign seats if the other passenger is on the same PNR. For some uber-important seats you both need status (row 1 springs to mind) but I've never had problems for things like upper deck seats.