I’ll answer what I can based on what I know. If somebody else corrects me, definitely take their word over mine though. Grammar is still a new wheelhouse for me.
Pg. 2
It is, but in conjunction with なんでも. By my understanding, one of なんでも 's meanings is “anything”, so なんでもありません is literally “anything doesn’t exist.”
“It’s nothing,” is how we would say that in English, though. You’ll also see it as なんでもない, since ない is the casual form of ありません.
So, I could be wrong, but my understanding is that katakana can often be used for emphasis in much the same way we use italics or bold print in English. I think that was what this was supposed to represent. I also think the translation you have there is off. I liked the interpretation given by @omk3 on post 14 of this discussion thread (how do we link, y’all, just realised I’m not sure how, hahah), since they worded it better than I could, I’ll point you there.
Pg. 4
So, でも has multiple uses. In this case, I think it’s meant to mean “or something”, as opposed to “but”.
“Are you practicing your faces/poses or something?”
Is how I interpreted the sentence.
This page explains uses of でも fairly well: How to use でも ( = demo) – Maggie Sensei
The rest of those questions I’m too foggy on to give a proper explanation for, so I will leave those to more experienced folks than me!