First of all, let me tell you how happy I am to see that this story isn't quite dead yet. In my opinion, it's one of the best ponyfics this fandom has produced, and certainly is one of the most underrated.
As for the reunion, I didn't feel it was out of place, provided there's more development of Trixie's motivations in subsequent chapters. Snails got what he wanted out of Trixie when she came back, so I see no reason why he wouldn't be happy to accept their current arrangement. Trixie's motivations are more opaque; it's hard to say how I feel about her actions without knowing what exactly drove her back. Was it simple loneliness? A sense of responsibility which she's beginning to develop towards Snails? The first inkling of a conscience? All of the above?
Now, I don't think those answers need to be retroactively added to what's already been written, but I think it is something that needs to be addressed at some point. Maybe not until several chapters hence, when (if?) Trixie's a bit more willing to open up to Snails. So far she's managed to tell him a fair bit about her past, but she's been much less forthcoming about her motivations, her wishes, her inner thoughts, etc.
As I said, I don't think there's any hurry to address that. Since Snails is the primary POV character, it's perfectly acceptable for the reader not to be privy to the impetuses for other characters' actions in advance.
The one thing I didn't like was Trixie's "You're the first pony in years I've actually been able to talk to," bit. I'm not sure whether it was supposed to be taken at face value, or if Trixie was just saying what she thought would get Snails to come with her (I took it the latter way when I first read the chapter, but on rereading I'm not so sure any more), but either way that paragraph did seem out of character. I think she could be a bit more restrained, and still convey to Snails that he is important to her.
I hope you're able to find a way to continue writing which satisfies your vision for the story. I, at least, will still be waiting when chapter 9. In the meantime, please feel free to e-mail me if there's anything I can do to help.