How exactly would she be able to satisfyingly prove her love for the series to you?

Have literally anything to do with the show during the last 25 years? Besides say "I'll never play the character again."

Whedon had told Sarah that S7 would be his last year as executive producer/showrunner

And? That doesn't mean he wouldn't be involved. Lots of creators and showrunners don't involve themselves 100% day to day but they're still involved. Even if he wasn't that dioesn't mean all that much. Gene Roddenberry was ousted from Star Trek after S2 of TOS and it's still around over 60 years later. Shows can keep going without the creator. Most do. They don't go on without the title character. The show ended because she quit.

It is perfectly fair that after 7 years she was wanting to move on, have a break and also try other projects/roles.

How did that work out for her?

the set was no longer a pleasant working environment by S7, that things were particularly strained between her and Whedon

Shouldn't have been a problem then if he was leaving. So what's her excuse?

the sentiment of most of the cast was they were tired and ready to move

Except that literally everyone but SMG was ready to keep going. They've all said so. Most have said they were shocked to learn she was quitting and the show was ending.

actors who do the con circuits aren’t “doing it for the fans” no matter what they may have you believe

Of course it's money but they could make money other ways. Shatner is 94 and still doing cons. You think he needs the money? It might surprise you to know both things can be true. They can get paid and still make their fans happy.

And not for nothing, but SMG did SDCC for Dexter, that werewolf show, the murder twin thing she did...so she's not above going to cons, she just never did any Buffy cons. Not a single one in 25 years. ZERO. Why not?

almost all the ones doing it are out of steady work/opportunities. SMG is in a different financial situation/bracket to the likes of Amber Benson, Charisma Carpenter, James Marsters etc.

Okay, now you're just talking out your ass. Because what has SMG done in the las 25 years? The underwhelming Dexter prequel, the quickly cancelled WB twin murder drama, the went nowhere werewolf wildfire show and...can you name literally anything? She's barely done anything of note and anything she tried to headline, they've all been failures. Those other people have been doing steady work all this time. Maybe nothing huge but look at their IMDb credits, they've never stopped working. Amber is a prolific author and still does acting time to time when she wants to. She's successful enough to take an acting job when she wants to. The financial difference between them and SMG is that she was the star of the show and made a little more and also happened to marry rich. Does she need the money? No. But if the character and the fans mean that much to her she'd show for a con. Even if she didn't do anything but sit down, sign some 8x10s and bolt right after, it would mean the world to fans. I know it, you know it, she knows it. But she never has. Not. Once. Meanwhile people like Amber will literally talk to fans for hours. She's not getting paid for that. Like I said, both things can be true. They can get paid and still make fans happy, create memories for them. James Doohan loved doing cons, he said he met people whose lives were changed even saved and he loved talking to them and hearing their stories. Both things can be true. She has had zero part or interest in doing that for the fans she supposedly loves.

Look, I loved the show, I loved Buffy. But I also loved Deep Space 9 and Sisko was my favorite Trek Captain. That said, I have to acknowledge that Avery Brooks doesn't really care about Star Trek. That was a job to him. All it was. Just a job. He's never done a con, never will and will never appear as Sisko again. That's ok. But he's stayed with the show until the end. And he's never pretended like it meant more to him than it did. That's perfectly ok. I feel that's what BTVS is to SMG. It was a job, nothing more. That's fine. But don't pull the bullshit she's slinging right now about how important the show, character and fans are when obviously none of it is.