Bryan Singer just tweeted a pic that pretty much proves they were planning on using Quicksilver since before the Whedon/Disney announcement.
The big question, is this a twin living on their own or both with their mother (if they don’t make her missing/dead as is canon) or with their adoptive parents? Seeing as the actor is in his late 20s I doubt he’s playing a 15 year old so not sure he’d be living with his parents as a 20-something. It could be his own house. And if it is, would Crystal be in the picture yet?
Is this why we haven’t heard anything about Wanda? Magneto goes to his son, who is living alone (or with Crystal), to get his help, so no sister. There was talk that the team will need Quicksilver’s speed for something specific. Does this mean that the two teams are working together? Or Charles knows about Magneto’s kids? Or they just know about a super speed mutant… and coincidences happen? So many options, some more awesome than others.
As for the Maximoff cold war, it takes time to set up a location for filming, you have scouting, then getting it prepped, etc. This sequence also had to be written and if they are bothering with doing outside shots then it may be more substantial than a quick scene. Not something to be likely be written on the fly. Lastly, the rumor after First Class was that this movie would be more Magneto-centric which this would play right into and show that Quicksilver was meant from the beginning.
Still no words on whether or not Wanda will be involved at all, but let’s hope she at least gets a mention…
X-Men: The (fan fic) Series 
How exactly does this prove he had quicksilver plans before Joss Whedon?
It’s basically a culmination of evidence. First you have all the rumors back after First Class which said the sequel would be Magneto-centric. Then the fact that Singer had an actor signed on only days after Whedon’s announcement. Now they have a set piece.
Filming only goes as fast as people think it does because of crap loads of pre-production work that is done before filming actually starts.
It takes time to do interviews then negotiate contracts. It takes time to scout locations, secure locations, and then prep them for filming. The fact that they are doing outsides shots of a home (which means likely internal shots) means that this wasn’t last minute as the production designer has been working on it for a couple weeks. The internal shows could also have already been filmed on a sound stage or whatever.
There is no way they just said “boom, make quicksilver in this, write something up” and got this all off the ground that quickly.