This whole “Stiles and Derek worked together over the summer” theory seems to be nothing more than another way for Sterek fans to remove Scott from the picture. Sterek doesn’t exist on its own. It never has and it never will. The one thing that connects Stiles to Derek and Derek to Stiles is Scott and it has always been Scott. To imply Stiles and Derek worked together over the summer without Scott is to completely ignore everything season one and two told us about Stiles and Derek’s relationship.
Do you think it’s a coincidence that the moment Scott was let in on the fact that Erica and Boyd were kidnapped, the boys started planning on how they were going to find the two and subsequently break into the vault? If Derek and Stiles worked together over the summer to find Boyd and Erica, why didn’t they find them? Why didn’t Stiles just go and begin researching like he did with Scott? Why didn’t Derek just go to Deacon and have him hypnotize Isaac? I’ll tell you why, because Sterek can’t happen unless Scott is involved. He is the connecting factor. Without Scott in the picture somehow, Sterek doesn’t exist/interact.
It just doesn’t make any sense.