That's what I get for reading quickly, I read marshal as grand marshal (the prestige class not the mythic class) - not used to the mythic rules.
Right -4, brain thinking of the difference between using heavy crossbows versus light, which is a -6 difference to hit. I'd still look at at least one light crossbow for the off-hand, considering that DEX is currently +5 to damage on a hit, deadly aim would be +4 (+8 at level 12), and a light crossbow does an average of 1 point less damage, then a +2 to hit with the heavy crossbow if using a light crossbow in the off-hand should usually result in more damage delivered even when targeting touch AC, and that doesn't even count the light crossbow in the off-hand having an effective +3 to hit. Not playing in your game, I don't know if the light crossbow is a good choice. The light crossbow isn't so good if you are in AoO range a lot, or range is often 85'-120' (165'-240' with distance enchant) where the light crossbow isn't in the first range increment.
As for the monkey belt, I think the standard upgrading magic items rules from magic item creation would apply. Since a belt is a slotted item there is no need to hassle with the +50% multiple similar abilities surcharge, and the price should be based on the price for similar items. A belt of incredible dexterity +2 is 4,ooo gold while a monkey belt which also grants +2 dexterity is 9,4oo gold so the tail and climbing bonuses are 5,4oo gold of the price and the +2 dexterity is 4,ooo gold of the price. A monkey belt which grants +4 dexterity should cost 21,4oo, 16,ooo from the +4 dexterity and 5,4oo from the tail and climbing bonuses. The might be confusion with the greater monkey belt costing 18,ooo while a belt of physical perfection granting +2 strength and dexterity is 10,ooo meaning the tail and climbing abilities of the greater monkey belt are 3,6oo gold more for the greater monkey belt - but the greater monkey belt provides an addition climb bonus above that provided by the monkey belt. Upgrading items is subject to GM approval, however, and a GM is free to change the price or prohibit certain changes as they feel appropriate.