If you are not bypassing the spell prereq for the item, you are forced to 'cast' it.
http://paizo.com/prd/magicItems/magicItemCreation.html
From the creating wonderous item section (similar text can be found in almost all other feat specific sections).
If spells are involved in the prerequisites for making the item, the
creator must have prepared the spells to be cast (or must know the
spells, in the case of a sorcerer or bard) but need not provide any
material components or focuses the spells require. The act of working
on the item triggers the prepared spells, making them unavailable for
casting during each day of the item's creation. (That is, those spell
slots are expended from the caster's currently prepared spells, just
as if they had been cast.)
so in your case, each day you spent crafting the belt, you would need to use a copy of the scroll or have someone around who can cast it for you. Otherwise, you have to add +5 to the DC of the check.
You're misusing the guidelines.
First, let's take a look at what the Pathfinder Magic Item Gold Piece Values section has to say on this issue. Turns out, it is specifically addressed.
The correct way to price an item is by comparing its abilities to similar items (see Magic Item Gold Piece Values), and only if there are no similar items should you use the pricing formulas to determine an approximate price for the item. If you discover a loophole that allows an item to have an ability for a much lower price than is given for a comparable item, the GM should require using the price of the item, as that is the standard cost for such an effect. Most of these loopholes stem from trying to get unlimited uses per day of a spell effect from the "command word" or "use-activated or continuous" lines of Table: Estimating Magic Item Gold Piece Values.
Emphasis mine. It then goes on to take Mage Armor as an example of this:
Patrick's wizard wants to create bracers with a continuous mage armor ability, granting the wearer a +4 armor bonus to AC. The formula indicates this would cost 2,000 gp (spell level 1, caster level 1). Jessica reminds him that bracers of armor +4 are priced at 16,000 gp and Patrick's bracers should have that price as well. Patrick agrees, and because he only has 2,000 gp to spend, he decides to spend 1,000 gp of that to craft bracers of armor +1 using the standard bracer prices.
So yes, your Amulet of Mage Armor and Shield is a textbook example of the guidelines gone bad.
This is understandable. The guidelines are famously weak, often breaking with casual use, as you have encountered. This precise case, however, was anticipated and addressed.
Best Answer
Yes you can always take 10 when not in combat or your life is not in danger.
They seem to be in order.
In the case of belts / headbands, this seems to be the case. Note that Weapons and Armor have different rules (3x modifier).
Take a look at the Cooperative Crafting text here. The basic answer is yes.
The details are:
Clearly if you wanted to do this with scrolls, you could, but you would need to expend the scroll every day you are crafting. At 150gp * 4 days, that's pretty expensive.