Cap on the amount of remaining aps

I suppose the best way to test that is to level up again and record AP gain. Then spend 5 APs or so, and try levelling again.

Or for all we know, maybe the amount of unused APs you have impacts MP growth.

Well, I can tell you that I have 19xx MP with HB and a MoN at level 154, and have only reset about 150mp tops (base). There is absolutely no way to get that much on a spearman lv 8x without massive INT gear usage, unless Spearmen get more MP per level than DKs/DrKs.