3U: discard Otawara, Soaring City: Return target artifact, creature, enchantment, or planeswalker to its owner’s hand. This ability costs {1} less to activate for each legendary creature you control.
The ability counts only creatures but not planeswalkers, so you should pay 3U.
"If a black or red permanent, spell, or card not on the battlefield would be put into a graveyard, exile it instead."
Well, the rule cares about if that item is going into a graveyard, but not how the item goes into the graveyard.
Sacrifice still makes Bloodghost put into a graveyard, so the...
It's probably a bug rather than a ruling issue.
From MTG judge:
https://blogs.magicjudges.org/ftw/returning-mechanics-review/convoke-delve-and-improvise/
"Because these are alternative ways to pay mana, they act on the final cost, after it’s locked in and after the player has activated mana...