I didn't realize they were negotiating. But I also just googled and Feige neither confirmed nor denied it. It's hard to say if they actually are negotiating, but the door is certainly open for interpretation.
But yeah it would be tough. Kind of the same reason the Halo movie never got off the ground.
I doubt that Marvel would confirm or deny things until something concrete happens. Unless they were to strike a deal, I doubt they would want anyone to know that they tried and failed.
Personally, I don't think the Spider-Man fits well. Where do you introduce him? Do you assume his storyline was ongoing while the avengers were tearing up New York? It just doesn't fit retroactively. And does he need another reboot or standalone film to bridge the gap... Seems like injustice to just introduce him in someone else's movie.
I would anticipate some 'shared footage' of his exploits in the two most recent Amazing films being shown, possibly being reviewed by Cap, Iron Man or both.
I think it would be interesting for them to use Spider-Man as a new jumping off point, or centric character for a new avengers.
I think it would be more of an 'appearance' type thing, much like they're doing with Hawkeye or the Hulk. No solo movies, just appearances in ensemble films.
Sony would likely continue to handle any and all solo stuff.
If they can get the money deals taken care of (a big if!), I think both sides could greatly benefit.
