Did what happened to Gwen Stacy meant to make place for Mary Jane?

This question is impossible to answer for sure because we don't know the minds of the film's creators. However as Chopper pointed out above there are contractual reasons why Emma Stone may have been "killed off" in the second film rather than any other sequel.

Comic book fans will tell you that this was the only way Gwen was always going to leave was by falling to her death (although in the comics MJ was introduced sooner). The death of Gwen was a major story for Peter Parker, it also harks back to the promise he made to her father in the first film. If you remember, he promised to stay away from Gwen so she wouldn't get hurt.

To answer your questions:

Mary Jane would be back on the next sequel

It's quite possible that they would cast an actress to play Mary Jane, this question discusses in detail why MJ was not included in this reboot.

They would not have to shoot Spider-man movies at/about Oxford or England

This is where my comic book knowledge starts to break down - I don't know if she ever intended to study in the UK in the original stories. My suggestion would be that given the budget for these films the location was unlikely to be a problem, however the sense of her leaving and starting a new life was simply meant to draw out the storyline between the pair and make her death even more tragic.

They both conspired to fake Gwen's death, to keep Gwen safe

It's certainly an interesting theory. Characters in the Marvel universe rarely stay dead. If Emma Stone was available for another spidey film then there are storylines in place to bring her back, I think it's more likely that they'd use an existing story rather than create their own. However again this is speculation without seeing a script!

Certainly Emma Stone only signed on for two films whereas Andrew Garfield signed for three, obviously they could have negotiated more with her but I'd imagine that would be a part of it.