The answer is in your answer: Everyone in that house DID think she was still in love with Spike.
You said it yourself-"Did think she was still". The key word here is "still". It's not "Why does everyone still think I'm in love", but "why does everyone think I'm still.
Well, that's assuming I believed reason #1, which I don't.
But let's say I did. If the Scoobies believed Buffy loved Spike, then in this episode they would believe she STILL loved Spike. Does their belief make it so? I don't see how the way she fit in the word "still" means she loved him. It just meant that THE SCOOBIES MAY HAVE THOUGHT she loved him before, and still loved him then.
I don't think it was an exaggeration, I think she said the truth without thinking about what she was going to say first. It slipped. Notice she doesn't hesitate and doesn't correct herself, doesn't make clear she meant something else.
If anything, I think this further proves that she didn't love him. All along, Buffy was vehement about NOT loving Spike. Whenever anyone asked, she would say, "No, I feel for him," etc. Had she actually let something slip, I think she would have been embarrassed, I think she would have covered it up or tried to convince them it was an exaggeration. She doesn't. Because it wasn't confessional in nature. IMO, it was just an exaggeration.
In "Dead Things" Tara asks Buffy if she loves Spike. Buffy doesn't answer. It might be anything, which means it could go either way, if she did or didn't love him. Also, there are points in season 7 where people say things. The First in Caleb's form in Chosen, he asks Buffy: "Then why aren't you asleep in your dead love's arms"? She doesn't say: "He is NOT my lover/boyfriend. In the past, many people thought Spike was Buffy's boyfriend, and she hurried to correct them that he is not. It's not the case here.
Doesn't Buffy answer, tearfully, with, "I'm using him"? She wasn't just eerily silent. To me, her not answering wasn't a confession. I took her response to mean, "No, I don't love him, I'm just using him."
Does Caleb say "dead love" or "dead lover"? It doesn't particularly matter, but "dead love" sounds weird to me. Anyway, "lover" can be someone of a sexual nature only. In fact, whenever I hear "lover", I think, sex partner. I don't think, boyfriend. I realize it can be used either way, but that's the point, it can be used either way.
In Conversations with Dead People, the vampire (cannot remember his name right now, gahh) asks Buffy, "Your last boyfriend...was he a vampire?" Buffy doesn't correct him, either. And...so what? Does "boyfriend" equate "in love with"? I know plenty of people who have dated someone they weren't in love with...if you could even call the Spuffy S6 relationship "dating."
You may think, because I'm arguing this, that I don't think Buffy loved Spike. That's not true. I think when she told him in Chosen that she loved him, she meant it. But I do NOT in any way think she loved him S6...I don't see how she possibly could have. That's why I argue the "still in love with him" point so strongly.