like literally the way the story reads up to this point is that Nirasaka intentionally sabotages the relationship between Fuuwa and Yukiko because she was rejected by her own childhood friend and wants Yukiko to choose hedonism over monogamy
That's one way to see it. Nirasaka does it because she had also a childhood friend who end up rejecting when she reveal she was a succubus so she fear that it happen to Yukiko too so she try to bring her on the same way because she doesn't want Yukiko to feel the same way as her. Bear in mind that Nirasaka doesn't know that Fuuwa know about Yukiko being a succubus so she can't know that Fuuwa is ok with it. Maybe she would stop bother Yukiko if she know about Fuuwa.
You also forget that Yukiko asked Nirasaka about being a succubus. ANyway, every succubus as their own way of eating life energy, as i remember beside Yukiko in the stall, iirc she does it by hug.