What is the basis for this in the films? It's obvious that we will need to throw out ancillary materials as part of this retcon....but as far as I can remember there isn't anything in the films that make it impossible for all of Phase 1 to happen in the same year.
Also, both Vision's comment and the "8 Years Later" in HC allow wayyyyy more felxibility than people are thinking, because they are both informal. Either date could actually mean anywhere from, say...7 years and 9 months to 8 years and 10 months in an informal context and still be true. That allows enough flexibility to cover all the way from November of say 2009 all the way to February of 2011, for example. That's the power of speaking informally.
What basis in the films? I've already commented on the IM3 issue. Looking at other films. In Avengers, it's stated both by Fury and by Black Widow that the events of Thor and Incredible Hulk happened (at least) a year prior (in fact, Black Widow states that it's been
more than a year since Banner's had an incident) IM2 takes place at the same time as Thor and the Incredible Hulk so that also takes place a year prior to Avengers. So there's those three movies and their time frame in comparison to Avengers.
Looking at IM2, the beginning of it tells us that it's been 6 months since Iron Man. Ergo... Avengers is a year and a half (at the least) after the events of the first Iron Man film. Iron Man, IM2, The Incredible Hulk, Thor and Avengers cannot all take place within the same year. The dialogue within the films alone tells us that.
And sorry... I could maybe give you the 7 years and 9 months being called 8 years, but if you're going to round that up to 8 years, then "informally" 8 years and 10 months would be more likely to get rounded up to 9 years (rather than being rounded down to 8) - people do, after all, tend to round up after the halfway point if they're going to round at all.
Also, in terms of the Civil War dialogue, I don't see Vision as being the type to speak 'informally' - out of all the characters, I think he's going to be more precise then most.
IMO, if it's a choice between taking Homecoming as fact and retconning almost the entire timeline set before it or else taking Homecoming (and even Vision's dialogue in Civil War) as being a writer's mistake... I'm more inclined to go with the writer's having just made a mistake/continuitiy error with Homecoming and Civil War (just my opinion)