I HONESTLY think, that Joss, upon reading all the demands the various PTB at WB wanted out of JL, stated "You know it would be easier if we just scrapped the whole thing, pushed the drop date a year ahead and started over" And was told no we are not doing that... One rumor was that various WB execs would be getting nice bonuses if the film dropped on time, hence the push to get it out that year, so yeah.

WB wanted that movie to premier in November 2017, wanted it to be less than two hours, wanted all the 'epicness' of Snyder's action shots, with Whedon's witty dialog. Oh and this has to be done in under 4 months. Like you said little wonder the set was a toxic hell hole.

And you are correct, shooting a VERY effects heavy movie on film is a bad call since you will have to convert to digital ANYWAY to insert the CGI in post, that costs time and money. So just shoot digital from the start making any future conversions/adding effects.