Unless specifically stated otherwise (like through Soul Exchange, for example), tributes must be from amongst cards you control. You can't tribute an opponent's monster unless a card specifically says you can.
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Yes, monsters may be 'tribute set'.
Yes, the monster obtained by the effect of Creature Swapcan be tributed.
You would miss the timing to activate the card as the monster sent to the Graveyard must be last thing to happen. During a Tribute Summon, the monsters are sent to the Graveyard, but the last thing to happen is no monster being Tribute Summoned.
Yes. Face-down monsters can be tributed for tribute summons, card effects, etc.
You need one tribute for a Lv5 or 6 monster, and two tributes for Lv7+, though many higher level monsters have additional or different requirements.