Disney isn't opposed to spending money to simply break even, since they look at the long term investment in their releases, not just how much they'll make during the initial box office run.
The problem I see is that I'm not sure an R-rated horror movie fits what Disney has planned for Marvel. Adding more gore and horror to make it "scary" is just a cheap way of making horror movies anyway.
Theoretically, Blumhouse mastered doing those types of movies for under $10 million and filming in less than 4 weeks in LA, so it's not necessarily expensive to reshoot. The question is what the greater strategy is. If it does end up R-rated, how do they deal with that on Disney+ since it looks like something for kids who watch Descendants but actually is a solid R-rated horror movie with a subplot with gay characters? Will parents complain?
The word of mouth and hype for the movie are so bad and the assumption is it's bad even when it really isn't, so at this point, can Marvel & Disney even turn it around? Even with a great script and if the reshoots are fantastic, have too many people already decided the movie is terrible and they aren't going to see it? Marvel could have tried to promote it at SDCC, and didn't. Maisie was there for the Game of Thrones panel and was there on Saturday supporting former co-stars Richard Madden (The Eternals) and Florence Pugh (Black Widow). Instead, Dr Strange 2 was promoted as the first Marvel horror movie and more bad press for New Mutants that it didn't even get a shout out from Kevin Feige.
If they wait until the reshoots are over (possibly in January or February) to do a new trailer, it's too late to turn impressions around, but no one at Fox is thinking that far ahead. And that may be the issue Disney has with Fox - they literally don't have any clue what they're doing.