[all costs per person]

+ As others mentioned: starting Amsterdam (or elsewhere that's even cheaper) with save you ~240 on the base fare (£1800 for LONE3 ex-LHR vs ~£1562 ex-AMS) and if you fly NYC-AMS (with no stopover) at the end then it will also save you ~£43 in taxes (for LHR-AMS in economy). If you instead do NYC-LON and then later LON-AMS as your final segment then of course you'll still pay this ~£43 tax and only save £240. That's still plenty enough for one or two trips between london and amsterdam.

+ If you wait till April 1st to ticket you'll save ~£50 on the JAL segments as fuel surcharges will drop. See: https://www.jal.co.jp/en/inter/fuel/detail_world.html [NOTE: you may need to wait till slightly after April 1st if ticketing via a different airline as I'm not convinced this reductions are picked up instantly]

+ I don't know the details for sure but as has been mentioned I believe ticketing on BA means paying higher fuel surcharges for segments by other airlines. I just always ticket on AA so avoid needing to know the specifics.

+ Do you value earning miles on any airlines? Clearly this itin does not maximise that. I value a mile at 1p and like seeing the odd new place just for fun if only for 8 hours so will generally add stops if I've got all the necessary cities in already. Beware taxes/fuel charges though of course.