If you're using a model then it isn't really editorial.  I know SS allows staged editorials nowadays but I'm not sure if it covers that situation.

Imagine this scenario: a model shot in Times Square or Piccadilly Circus, with hoardings in the background. Wouldn't that require both?

Once it's editorial the model release is irrelevant.
If it's released and the model is the subject, then the background is ancillary.

Depending on the background, it may need property releases if it is going to be used commercially.
But as you say, if it's editorial, no MR is required, in most countries. (France is iffy, and I believe I read something about either Romania or Hungary, I forget which.)

For iS, there is a specific article re models in real situations with logos etc. But that won't apply to other agencies. http://www.istockphoto.com/article_view.php?ID=1736