jaarrg
To answer your questions in order-
1-LGW contracts are the same as LHR until you are 17 years in the company when the payscale at LGW goes flat- ie no more annual increments.
2-You can always ask for a later start date/different base but its a risky thing to do. Seniority counts for everything in BA, a few months difference in joining date could make many years difference later in your career when it comes to command/fleet transfers/etc. Also if the economy takes a turn for the worse and BA stop hiring,they will be under no obligation to offer you a course. Various rumours doing the rounds about grounding older aeroplanes if the oil price stays high. BA is as secure a job as you will find in the UK, I would say get in while you have the chance.
3-Your initial ('engagement') freeze is 5 years regardless of previous type rating. The Airbus is not a shrinking fleet so you are likely to stay on it for as long as they need you there- ie the full 5 years. You could start at LGW then ask for a base transfer, technically there would be no training cost. Not sure about the rules on that.
That said, Gatwick is a nice base to operate from, the rostering system is better (than LHR's) if you are junior. We have commuters from all over the UK and Europe who all seem to make it work for them.
Cheers and congratulations on getting in!