No
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No part of Call of the Haunted or Royal Decree affect a monster's ability to attack. The Sangan will be able to attack as normal.
Yes, Sangan's mandatory trigger will activate after the monster is summoned.
Yes, Sangan has a Mandatory Trigger (so never misses the timing), and a tribute is a form of Send. If you tribute it for a tribute summon, Sangan will activate when the summon is successful, or if the summon is negated, after the resolution of the negation card. If you tribute it for a card effect such as Enemy Controller, then allow the chain to resolve. Sangan will start a new chain as chain link 1.
Sangan triggers in the graveyard, so it's irrelevant if it was face down or face up when it was destroyed. It will trigger in both cases.