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In regards to the orb question, you can raise Mai's lewdness by interacting with those red flowers scattered across maps. Ten lewd points levels her up to the next lewd level. Lewd skills are unlocked as inspirations in battle, when you are both over a certain lewd level and (if I remember correctly) are fighting in the nude. After you get the orb from the pyramid, ideally rescuing the strip club owner while you are there, you need to go to Poire's old house to get the outfit hidden there.
In regards to the sewer question - I'm sorry to say it's been a long time since I played THM. The guide post seems to indicate you need to have the non-virgin ending and then return to Merlan. I opened the game in RPGMaker, but unfortunately couldn't find the details of the event that triggers the spirit going away immediately. The guide post seems to indicate that Mai's gotta be a non-virgin to get through, though.