About point1, i heard someone said the Sheeda did save Oguma very long time ago and this was the reason why Oguma protected her and her lovers at the cost of his life.
about point 2, I really thought he tried to but was unable to fight to full extent, like what you've mentioned he didn't know what to do after winning the fight and most probably he had to kill everyone again: including Nina.
point 3, refer to the mystery of emblem conversation Nina refused to get marry at first but Boa persudaed her.
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