Iron Man effect is only true for the opening. Once people actually saw the movie and word got out about how his role is barely in the movie, do you think people seriously kept going to watch it just to see this tiny role of Iron Man? Nope.
Do you seriously think that Homecoming would have pulled down those numbers worldwide if Iron Man wasn't in it and featured so prominently in the marketing?
BvS would be easy 1 billion if it had come out after Wonder Woman. It is not like the story in WW makes any less sense this way since it is a Super Hero Origin story. Heck if this happened, her cameo would have been a BIG deal in the actual movie but ended up being a silent thud.
I think the issue with BvS is that it was the first time that Batman and Superman had ever been on the big screen together (and the first time Wonder Woman had been on the big screen period).
We just had Civil War last year, and RDJ was in Spider-Man Homecoming for like 5-10 minutes. Happy had as much screen time.