Even though Snape had to give excuses for all his other line-walking, strangely, that one never came up. Again, I figured either Rowling completely forgot that plot point (and that's very possible), or she was dropping a hint to the readers as to whose side Snape was really on.
Well, I think it never came up because there was no way for Voldemort to know that Snape alerted the Order. The concern, from Snape's point of view (had he been a double agent), would have been Harry and Dumbledore discussing the issue. Really, this type of incident would be of great value to a double agent, as it would be a perfect opportunity to erase all doubts in the minds of the Order, and perhaps even in Harry's mind, while at the same time it was unlikely to cause any serious damage to Voldemort's cause.
Also, as I recall, Snape had nothing but contempt for Umbridge, so there is also that aspect to consider.
The Unbreakable Vow incident interests me in part because I think it actually strengthens the eventual reveal quite a bit, not because it makes it surprising, but because it increases the degree of difficulty, as does Snape killing Dumbledore. There has to be a satisfying explanation, and there is: Dumbledore wanted Snape to end his life. I don't like the seventh book of the series at all, but I think the Snape subplot actually works.