I thought that part in the walkthrough didn't seem right since that's not how I got those endings. I was gonna mention that in my comments but it's already handled X3. Anyways! Onto the question answers.
1. Did you enjoy the story and characters?
Yes, Mike and Sarah were fun as individuals and as a couple. And I imagine it's rather hard to have the same person carry a conversation with themselves but you did a really good job. As for the story-
Stories like this can be hard to do with multiple paths because there is generally just one right answer; one is fake and one is real, they're both fake, they're both somehow real. Once you know the answer the story can struggle to know where to go with other paths. This game does have one truth (with one being real and one being fake) but changing which one was real was a really neat way to keep the paths interesting. It was also fun having to first worry about making sure you were real and then still having to convince Sarah of that.
I am a little confused as to why, in one ending, the fake talked about Mike seemingly having violent thoughts deep down when that didn't really amount to anything/show up anywhere else and ultimately real Mike is supposed to be pure of heart. If the fake was just trying to get the real Mike to keep attacking it can be a little confusing for players since it's never directly said.
2. How was the pacing of the story?
I'm not very good at judging pacing >.>. For the most part I'd say it was good.
Sarah getting past the two Mike thing did go a little quick since there wasn't much dialog between her leaving and coming back. Maybe just adding a few '...'s and/or fading out and back in to show that some time had passed would help? And I do think that it seemed odd for Sarah to pick a fake in some endings. In quite a few endings she is able to figure out the fake for sure so it makes sense that she'd single him out. But then in other endings she really didn't know but picked anyway. That didn't make much sense since, as far as they knew, there was no rush. So singling one out, even temporarily, didn't seem like a wise choice.
3. Which ending did you like the most?
I liked 1 & 7 the best, personally, cause they worked out the best for the people involved. Though 6 was also pretty cool because when I got that I hadn't found out that you the player could also be the fake depending on how it went. At the time I had only been the real Mike.
4. Did the story flow naturally to the ending?
Other than those ends where Sarah picked a Mike too soon I'd say so.
5. Any general comments?
I really liked it
. The character designs and expressions were fantastic-
especially all the fake Mike expressions
Overall I just though it all came together great and I'm really looking forward to your future projects :3.