HI, I'm new and have two questions I cant find answers to in relevant threads (although in fairness I've only been going through them for about three and a half hours..)

1) How far back can you add miles to BA EC? And what proof would you need? (For example I remember travelling AER LINGUS from JFK to Dublin in Fisrt/Club and back three years ago and never put them on. Are they lost for good?) I have no remaining paperwork, but thought perhaps my presence on those flights might be documented somewhere. am I kidding myself?

2) I am BA GOLD based out of Europe and already have 900 tier points for this year, so am already over my necessary quota to stay gold next year. I have booked a flight in CW over Christmas from LAX to LHR and back. Is there any reason at all, points wise, for me to travel BA? Would it not be more interesting for me now to fly United, say where I am Premier and almost Premier Exec, or is there some use for those extra BA tier points I dont know about?