It has shit ratings because it's on a premium channel, and actually currently gets the best ratings of any other show on that channel and is their flagship cartoon. Disney typically doesn't do a lot of merchandise for shows on that channel, but there's more than "zero" merchandise since it has books and shit.
So that actually doesn't indicate anything since it's totally normal for that channel, and while I think Nefcy being a woman might be how it got made in the first place I don't think it's why it's successful now. I think they were prepared to give it just 13 episodes (unusually short) but they renewed it because it had a good premiere and got a lot of fan attention.
Since Disney approves of what happens in the show in the first place, they couldn't get pissed off by something already on air. S3 is totally written by now; S4 production began a few months ago.
Disney themselves would be the ones who chose to advertise the movie with "Toffee had killed my mother," not the crew.
Not that any of this has anything to do with whether Moon will die. I just think your points are off.