MTB to Tourer Fork Geometry

My understanding is that when ridden on flats, those Rockshox would probably have been compressed to somewhere closer to the LHT's measurement, but I'm wary of shortening the fork length because touchier handling and a crank closer to the ground and / or panniers wouldn't be ideal.

You're referring to sag which would mean that yes, while being ridden the a2c of the rockshox fork would be <400mm.

I'd not be entirely convinced that the bike's design would have considered sag though, ie headtube angle etc was probably set for a2c of 400mm so you should probably - as Skully suggests - look for a 400mm fork.