Yes but also no: in Roya, you can do exactly that and there's nothing to stop you since afaik anal acts don't really have higher lewdness requirements than vaginal penetration acts. But then the Earth corruption will be bottlenecked sometime into chapter 2 out of 3, leaving you unable to spend Corruption. This would only work on the lowest difficulty setting where you can't get a Soul Break from excessive stored Corruption (and I'm guessing all that Corruption would wreck Aura's Willpower, making it hard to play even on that setting, though Blessed Waters could help). Of course, this also stops the linear Earth corruption arc less than halfway through, so you'd only have Roya.
As for the other question, there are a couple of mental changes that lower Lewdness requirements across the board, but I don't think there are any for specific fetishes. Though there is something like "fetish experience" where the higher it gets, the harder it is for Aura to refuse a proposition with those fetishes, if she meets the lewdness req.
You do get to choose who gets MC's virginity. The event chain that leads to sex with the boyfriend is in the Values room in the second chamber, which you can start whenever you want once you have 20 Lewdness and some Corruption. It takes 30 to get to the vaginal sex with him, which is also the minimum requirement for vag sex with others in Roya. So if you want someone other than the BF to be first, just do their scene before touching the Hedonism orbs (which are the bottleneck I mentioned).
That said, the H is actually one of the weaker aspects of the game, fairly tame and very disprersed across the game (dozens of scenes but also dozens of hours playtime), so I wouldn't compare it to a gangbang simulator like KP.
