It may seem obvious to some, but someone like five pages ago said something that resonated with me and pretty much summarized the Glazers problem, and it's that they don't care about the football aspect enough to want to win at it. Which is what a big team like United ultimately needs.
City owners cheated concerning transfer rules. To win. So did Barcelona's for years in payments to the refs. Real Madrid owners are actively suing UEFA and keeping the threat to sue another 10 of their european rivals, esentially because they want the rules changed so they can win more. These are extreme examples for sure, but they point to the same direction. Do you imagine the Glazers trying to cheat or at least work the system in United's favour? The only thing I can think of is the leveraged buyout which was Malcom's move and it was about working the system in order to take profit from United to themselves.
An interesting dimension of this is that even when the Glazers don't know/don't care about the football side as long as they make money (because yes, they do actually try and invest United's money when they perceive their yearly dividends are at stake), they are sensible to the press and social media's general opinion. Two of the last big decisions they had to make were actually changed by public/fans opinion even when they meant less potential money: the Super League and the Greenwood situation. In both cases, the owners perceived the backlash to be worse than the potential benefit and backed down.
That in itself is a hint that protesting at some level does work. Problem is, the portion of fans willing to take action will never be more than 10%, maybe 20% of total fans. Which is not enough to take extreme measures that actually work. The other 80-90% don't really care enough to be inconvenienced by sitting out matches, stop buying t-shirts and merch or turn off the TV for what is probably one of their few escapes from the hardships of daily life. Which is why the protesters have to play it smart in order to have a chance at influencing the decisions.
Regarding the sale I don't think the decision is going to be influenced by optics so much as by money and by sentiment (this is Malcom's greatest gift to their sons after all, and they probably know they can't make something as profitable by themselves so they have problems letting go). That explains why they are dragging their feet and extending the deadline for as long as they can. But eventually they won't be able to do so without pissing off most of their future partners and allies (Qatar, INEOS, Rayne, banks, smaller investors, etc), and that will be the decisive moment.
Regarding moral qualms, United's already been open to billionaires money for decades and 99% of it is either blood money or have been touched by blood money in the past, so IMO that ship has pretty much sailed. Plus, from my understanding football has been used as a social/political tool from its origins, with the process accelerating between the late 70s and early 00s particularly in Europe and in the UK. Professional football's current state (with consortiums, state clubs, sportswashing, oil/blood/illegal money pretty much everywhere) is at its core a consequence of that. So in the end where to draw the line in the sand depends on every single fan/supporter, knowing there's a chance it has to be redrawn anyway because every one of us likely love the sport more than the hate/repulsion we feel for the cnuts who have been ruining it for ages. It's a sad state of affairs but it's the hand we're dealing with.
TLDR: Glazers don't care enough to win. They do care about backlash though, so protests work until a certain point. They won't in this case as there are bigger issues at stake, but since they started this process their days are numbered anyway. Thing is, football is rotten at its core anyway no matter who the new owner is, so as a fan you can choose the hill to die on and that's it. And hope the new owner wants to actually win on the field, of course.