Since the Boss Monster is Level 20, if no one adds another Monster with
Wandering Monster, or enhances the Monster(s), then you're going to win with that combination. The Boss Monster is not a protected combat, and anyone adjacent to the Entrance can throw Potions and the like, anyone can play Monster Enhancers on the Boss Monster, and anyone can use Wandering Monster to add a Monster to the combat (and play Monster Enhancers on those Monsters, too).
As for your other questions:
- There is no limit to extra Hands you can have, however you have to be able to use the card that gives you the extra Hand. For example, you wouldn't be able to use another Headgear that gives you an extra Hand if you're already using Arm-y Helmet (unless some other card allows you to have a second Headgear).
- The rules (p. 16) say that if an Items isn't a Worn Item or doesn't need a Hand, then it goes on your belt. Worn Items are Items that go in your Item slots (Armor, Headgear, Footgear), Clothing, and Shinies. No Item will say it's Worn, but if it says one of the five things I mentioned above, it's Worn.