Custom items questions

1. What is the cap for massive criticals upgrade? I just don't believe that a weapon could be upgraded to 20d12 mc.
2. Can I make a custom "Everfull quiver of arrows"? And if I can, what is the type of that item?
3. Can I make DM upgrades for quiver as a weapon? (damage types, enchantment upgrades etc)

I hadn't thought of a cap for massive criticals.  Since the range for them has been greatly extended through the CEP, let's have a max value top out at 4dX, X being the base dice used for that weapon type...d8 for longsword, for instance (use d12s for greatsword).

The everfull quiver is a scripted item and cannot be used as a custom reward item.

DMs can not directly upgrade an everfull quiver.  All the upgrades that can be done to it (via elemental crystals and such) are specially scripted, recording specific data on the item that is used by the restocking scripts to apply properties to the ammunition that is created.