The reason Fox kept bumping it was because there wasn't a good comparable movie to show how much a PG-13 horror superhero movie will make and the comps were lower than they would like. While some horror movies have done well, most open in the $15M- $20M OW range and do poorly overseas. That wasn't seen as enough and Fox execs lacked confidence in it making enough money and wanted to be the person who overhauled it so it opened bigger.
The reason Disney bumped from August 2019 to April 2020 was during an overhaul of their entire release schedule with probably 20 different changes. Both New Mutants (Fox) and Artemis Fowl (Disney) were bumped. Some of the changes were to move Fox movies away from Disney movies to allow the Disney movies to do better (Call of the Wild was victim to that especially). But it had to do with the fact that Disney only was accustomed to a release schedule of one movie per month, maybe two occasionally, and where going to have to gear up.
Fox had let go most of their marketing execs. They were bringing in contractors to run campaigns and that was a disorganized mess. Disney was in the process of getting out of Fox contracts with marketing vendors and bringing all of Fox marketing into their vendors - who didn't have capacity to do that number of tv and digital ads.
Had New Mutants kept that 8/2/19 spot, then there would have needed to be a large push for it at SDCC but they weren't ready for that - including how Marvel Studios wanted to deal with mutants. Imagine the cast doing press junkets in late July and being asked about it.
Whether a movie is or isn't good doesn't really matter in marketing. Great movies under-perform all the time and the success of a campaign has more to do with getting people to see bad movies than marketing good ones
Disney doesn't dump movies. And Disney isn't treating it as a horror movie. It's a superhero movie being released in April. Artemis is going to see a very, very big push.
The week NM is getting releases has the largest percentage of high schools on spring break. They could have moved it up when Godzilla vs Kong was delayed, but they chose not to. They wanted that spring break date and it actually might cost Mulan.
The live action Mulan is going to be tricky because it's not as family friendly as the animated version. It's made for China. It's going to aim more for teens, but will they be interested? New Mutants is going to be targeted at a younger audience then probably most of you expect and appeal more to a male audience - which Mulan might not work. I have a hunch Mulan will appeal more to a female audience and probably older than Disney would like (90s kids nostalgia).
While Bond will have some pre-teen and teen audience, New Mutants has its niche. It's an older audience than Peter Rabbit 2, more male than Mulan and younger than Bond.