Is there a reason why he couldn't just use a fake mustache in the other film? I mean it would have had to look better than the CGI job they did.
When Justice League reshoots became a real thing because WB execs didn't like Snyder's rolls (they felt it was "too grim and depressing" based on test projections or not, who knows really) most other actors were already working on other movies.
Then we've heard Henry Cavill had somewhat a moustache contract for Impossible Mission 6, like he couldn't shave it during filming and production / promotion. Maybe it's bullshit (I can't be arsed going back filming that before we discuss my bonus), maybe it's actual contract, who knows.
At the time Justice League was already in development hell, Snyder got a family drama (his daugher died), WB execs didn't have a clue about what to do with DC Comics then and their "Marvel but more serious and dark" method was already highly criticized after Batman versus Superman box office low profile.
Then some Hollywood people (agencies reps, etc) were like, "we already had a dark Batman with Nolan", and while Man of Steel is a decent movie, it kind of underperformed box office wise. In EU no one gives a flying fuck about Aquaman and Flash already got his CW TV show. What gives ? At this precise point WB response was to emphasize "how dark and grim" the DC universe should be because superheroes.
Hence Joss Whedon. Brought up by WB execs to (sic) "marvelize the movie" with quips, humour and iconic scenes. At this point, with most likely a hard deadline, he did reshoots on the run with some actors and spent a lot of time cutting Snyder's bits with rushed CGI and such. Also we have to know writers at this point of production are as consulted as you'd want to eat fish and ask ocean if you own them a tax.
On top of that they had to deal with
Ben Affleck suffering from post-Weinstein scandal, Aquaman actor
doing a rape joke, then Ben Affleck again
doing a joke about harassment. It's a PR nightmare, you have to strong arm interviews, corral actors, deal with press embargo and former DC movies reputation.
I think Justice League suffers from 1960 Hollywood production business model where studio execs basically tell everyone when to breathe, meddling with creative just because they wanna make a dent at Marvel and / or making a point on their curriculum. In the superhero moviedom, or as some call it "capeshit", nobody wants to make a point, they just want to make money.
