From the rules:
Quote:
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Originally Posted by The Rules, One-Shot Treasures
Most of these are used during combat to strengthen the munchkins or the monsters, and may be played from your hand or from the table.
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So, yes, you can use a one-shot Item you have carried on the table. As for turning them sideways, there's been no official declaration that one-shot Items must be turned sideways, as they need to be added to the combat to count. I usually turn them sideways, personally, but in the past, Andrew has stated that's not a necessity.
As for your additional question: Please don't tack on unrelated questions in the future, so as to prevent tangents and splits in the conversation. As for the answer: Yes. That's the whole point the rules try to make by saying that you can carry any number of small Items and that Items you can't use must be turned sideways. You can use two Hands of Items, but you could carry any number of Hand Items. You can use one Armor Items, but you can carry any number of Armor Items, and so on. Obviously, the one Big Item rule sticks, so you couldn't use a Big Item and carry a second Big Item.