My thought would be that exposure to finding problems hard is important, far more important than getting through any particular material, so yes, I'd notionally put that in first and then see what else there was space for. Depending on personality and what you use for that and maybe other stuff too, you might find your DS is happy to spend all the time that seems reasonable to spend on maths on the hard problem stuff, or you might find that his tolerance for that is initially limited and it's good to have some more routine stuff in the mix too.
Do you have complete flexibility about when to start classes, and can you let them go to the backburner for a bit without penalty if you like? E.g., suppose your DS started a K12Inc course now and then hadn't finished it when the AoPS AMC8 course starts in March, could he temporarily drop the K12Inc work for the duration of the AoPS course, if it wasn't convenient to keep both up? If so, I don't see a downside to starting all the hares running and just playing it by ear. If, on the other hand, once you start the next K12Inc course you need to keep at it consistently, then I'd be inclined to agree that taking a break to work on Alcumus and AMC8 stuff now might well be sensible.
The thing I find helpful to remember is "he has time". If you take a decision that turns out not to be absolutely optimal - e.g., he does something he turns out not really to be ready for, so that he forgets it and needs to do it again later - no big deal.
