Don't understand the complaint about lack of danger, but I'll leave you guys at it.
I'm here for the action though, not for the horror. If you're here for the horror, then yeah I can see a bit of that.
As for Mumei vs. Ikoma and their infections, keep in mind that Mumei is exerting a lot of more energy than Ikoma in these fights... further more she's removing the limiter (the ribbon) from her neck, something that apparently keeps the virus somewhat in check. But as that one scene showed, emotional distress/anxiety might also cause some effect.
As for Ikoma, since he hanged himself, he's always had that collar around his neck keeping pressure. So we don't know what effect that has on his status and what would happen to him if he removes it.
I'm not discounting that Ikoma might be a bit different, but the evidence as has been presented so far (particularly once you consider his reaction when he got hungry) doesn't lend truth to that idea.
I have no problem with Mumei closing herself off. Ever since the beginning it's been very evident that she has a very narrow vision when it concerns her "brother" and her mission. This wasn't out of nowhere, and seeing a reminder to people who have been cast-off merely brought her back to her natural stance, with a bit of distress of course when she began considering that she was distracted. First episode she ignored everyone and went ahead and did as she pleased. That she opened up a bit in an episode or two, doesn't mean that she can simply waive off the anxiety she's always shown when her "brother" is concerned.
I mean, has there been a single episode so far in which she didn't do as she wanted? That's pretty much her standard operating procedure. Not sure why anyone would be surprised by it other than her being a bit more cold and dismissive of everyone, which is fine. Everyone has moods.