Your original post stated that it was harder for the writer to ensure quality and spots mistakes than for artist. I say that quality control in art is in no way shape or form "easier". There are many mistakes like overcomplicated page layout, cluttered panels, inconsistent character design etc. that artists can't spot easily and obfuscate the meaning of the story. So to reiterate, no, art is not easier than writing.
I don't think Marvel gives any less promotion to ANXF than they do their other "off-shoot" titles like X-Force, Secret Avengers, etc. Also I think you're overestimating the influence of ads and articles in getting new readership. Companies know that word of mouth marketing is actually a lot more valuable than straight up advertising. Personally, it doesn't matter how many times I see Spiderman store displays or how often Marvel preaches the awesomeness of Spiderman, I still won't read Spiderman. Now if I read a phenomenal review of a Spiderman book, I might give it a try...Actually I probably wouldn't. A friend would have to shove the book in my head and threaten me to read it or else, that's how disinterested I am in the character.
The point is, the last X-Factor found its niche without the help of ads and what have you, surely this new iteration can do the same.
We already know the details in the Wanda and Lorna meeting: it's a girl's night out, which can only mean it's fun issue with no world-shattering repercussions for the Marvel U, ergo not something a casual fan would be particularly interested in. And just because they're sisters doesn't make it intrinsically more interesting than relationship between other family members. Granted it's unusual in the comic-verse, but I don't see how anybody (who aren't already interested in the characters) getting all excited about two women who share a father having a girl's night out.
First of all, Frozen is a Disney movie aimed at children, so I seriously doubt parents take into consideration whether or not there are siblings / families in the movie before deciding to take their kids to the theater. It's more of a matter "Hey a new Disney animated movie is out. That'll entertain the kids for two hours". The people that do care about inclusion of sisters because it's symbolic of some feminist issue, they're not the target audience.
Secondly, the articles that came after the film was released, that's not a promotion on the part of Disney. That's word of mouth from people.
Third, a movie about brothers can be just as compelling and successful even if you can't attach a social movement to it.