So after seeing the sneak peeks and all this stuff, here's a newer, maybe more realistic speculation:
Adam an Carly meet by chatting at Splashface(the facebook version). She really likes his personality (and his profil pic). After a short time, they begin to video chat very often, and Carly finds out that Adam doesn't know about iCarly. So she tells him. He watches one episode and thinks that it is really funny. He also checks a few iCarly-Forums and finds out about the whole Seddie/Creddie stuff. Right the next time they video chat he asks her about it and she tells him that it's not true. He believes her and asks her to go out - The stupid thing is, that he asks her for the day she goes to Webicon with Sam and Freddie. She tells him, and after a talk about it, they just decide to meet at Webicon.
The fan's acting is really, really stupid. They only care about who dates of the iCarly cast. After Carly let them ask some questions, she knew: ~Half of the wanted her and Freddie to come together, the rest Sam and Freddie.
Adam slowly starts to believe that Freddie and Carly are really dating. After Sam said it (who just wanted to start a riot because she thought it was funny watching all the people fight like crazy) he's even more sure about it. Adam tells Carly, that she should proof that Freddie and her aren't dating.
Carky gets extremely mad at Sam, who didn't know about Adam until now. "I'm sorry, Carly, I didn't know that.. That was just for fun." "Just for fun? You destroyed the chances that Adam and me could ever be a couple!" (Freddie watches them both not knowing what to say) (Sam:)"You could have told me before. If you wouldnt have been quiet like a rock I'd know now and this all wouldn't have happened." "Oh come on, Sam. Now don't try to blame me, You know very good that it's YOUR fault!" "And what shall I do now?" (Carly:) "Just.. Fix this." "How? Shall I tell Adam that you aren't dating? He won't believe me, and you know that. If he doesn't believe you he won't believe me." "Then proof it to him.." (Sam:) "Wait.. how?" In this moment, Carly looks at first at Sam, then and Freddie. "NO WAY!" both of them shout together. (Sam:)"Dude, don't imitate what I'm saying.. But, no!" "I agree." (Carly:) "Be a really good friend for once. PLEASE." (Freddie:)"O..Okay.." Sam doesn't answer. Carly looks at her with deep eyes. Sam just looks at the ground, thinking "No.. Sorry, but I can't kiss that... dork." (Carly, angry:)"Well, you didn't have any problems kissing him the last time you did, did you? __...__ Our turn is in five minutes again. Let's end this place I call not really happily a seconds hell. Then we can go home." In this moment Carly turns her back and goes away. Freddie looks at Sam. "Come on.. Do it for Carly, not for me.. Adam seems really important to her.." Sam doesn't answer. After one minute staring at her, he follows Carly to Webicon. Sam stands there until it seems that she'll come to late, then she runs to them inthe last few seconds.
Again, it's iCarly's turn and the fans ask questions like "Since when are Freddie and Carly a couple?" Sam gets kind of angry. She cries loudly: "..Okay, I lied. They aren't dating." A fan shouts: "Don't lie, you admitted it by yourself! CREDDIE ALL THE WAY!" Adam looks extremely disappointed. In this moment Carly looks angry at her. "Happy with what you did?" Sam looks away. In this moment Freddie touches her back and they both look at each other. "Come on.. Just for Carly" he whispers. In this moment, she answers with a weak "okay.." And Freddie starts to kiss her. In this moment they start to kiss in front of all the fans, also in front of Carly. Carly seems schocked and happy at the same time. "HAHA what did we all say? SEDDIE FOR THE WIN. GET THAT, Creddiers!" - A Seddier cries, and all the other Seddiers just cheer and look at them happy. WhenSam and Freddie stopped kissing, Sam looked into the crowd again and cried: "Proof enough?" Then she looked a moment at Adam, and said, a little bit less noisy again: "Proof enough?" He smiled happily.
Suddenly Adam doesn't want to date Carly, even after knowing that she isn't dating Freddie. "Sorry, my ex messaged me.. She says that she can't be without me. I can't date a new girl now. Sorry!", with this words he left Webicon. "What an idiot!", Sam says. (Carly:)"It's okay. Still.. Thank you Sam." "No prob.. Now let's get outta here. just get Spence." "Yea, your right."
Meanwhile Spencer has an "epic" fight with Aspartamay. Because of the fact, that Spencer just doesn't want to stop the fight, Carly tells Sam and Freddie to go with the next train, and she'll follow them with Spencer later.
Sam and Freddie go home. At first, Freddie follows Sam to her house because it's closer, then he'll go home on his own. On the way, they start to speak. (Freddie:)"Wow, Webicon was pretty.. Creepy." "..Right.. The only things the fans cared about was who should date who. I wish that wouldn't be like that.", Sam answered. Freddie mentioned: "I know. I mean, we should date who we want to date and who we are happy with, not because we look cute or.. funny or.. I don't know, hot together. You know.. That.. "Seddie"-stuff.." "Yes.." (Freddie:) "Adam was a jerk too." "Right.. At least Carly didn't cry for him. And she thanked us for.. You know.." "..Our kiss. Yes.. You know, it's funny how you never tease me after we two kissed for hours (makes a =P face)." "Haha, shut up dork.. Or I'll really hurt you." "o..Okay.(smiles amused)" "Okay, there's my house.. I gotta go home.. Good niht, Freddie.." "Good night.. Ohh, and by the way.. I hate you (said as amused as in iKiss)." "Haha. Hate you too."
Ending: Carly and Spencer walk home, Spencer still wearing the kind of embarassing costume. The people at the street laugh about them, but Carly tries to ignore them, until the episode end with Carly shouting: "JUST TAKE THIS FLIPPIN' COSTUME OFF; YOUR WEARING NORMAL CLOTHES UNDER IT!" And Spencer answering "NEVAAAA!"