I think the key word is just that. What Shagnasty revealed to Thomas was that his 'friendly neighborhood vampire' approach was an illusion, that inside, he was still what he was, and that he had been that all along. His relationship with Harry, and his attitude, have returned to more or less what they were before, but now the presence of that demon has been revealed, it's been revealed that Thomas is a danger to mortals around him, that the demonic part of him likes being a predator, it's always there.
Thomas had been telling himself, before, that maybe he could change, stop being what he is, Shagnasty took that illusion away. Now he continues to try to be decent, to be safe...but the menace is never far away.
JB had to go back to at least a similar relationship to what they had before, if Thomas had kept on as he was in the immediate aftermath of the Shagnasty affair, well...that story only goes one place and it gets there fairly quickly. If they had gone down that road, Harry, or someone, would have had to terminate Thomas, or else he'd have become a full-on enemy and the brother-relationship would just be gone.
But the danger that Thomas represents, the nastiness that is the White Court with the facade torn aside, now it's out there. I think part of what JB was doing, in Turn Coat, was 'deconstructing' the then-popular idea of the Romantic Tragic Vampire, both with Thomas and his victims, and with the interaction of Lara and Madeline.
Maybe, but Harry hasn't come to grips with it either. Now it may come to a head in Twelve Months, because unlike her little brother, Lara has never fought what she was. Actually she seem pleased with the results of Shaggy's "treatment" of Thomas, in her eyes he was now acting normal. If the marriage between her and Harry doesn't happen, this will be one reason behind it.
I disagree, I think the story could have gone on for some time, more complex certainly, but it didn't mean that Thomas would die or have to be killed.
Here is a further WAG, that this little problem will come to a head sooner than later. Harry will come to realize that Eb was right about vampires, they cannot change who they are. However, then what? Harry still loves his brother.