It's possible. But he'd be risking someone flatly contradicting him the moment he made his statement about the testimony - "no, you didn't listen correctly, she didn't really say that". And afterwards, of course, there's no point for him to go back in time because he's received evidence that she did not in fact testify as he wished.
Your scheme would work a lot better if he'd just listened to her testimony. Then he would know what he had to go back in time to cause, regardless of the way he used her testimony now. (grin)
He would risk it the same way he risked not actually being found by a teacher.
Sure, that would be the smarter thing to do, but then it wouldn't come as a surprise to the audience. This way it gives us and Harry a puzzle.
EDIT: New discussion thread here.
This is a new thread to discuss Eliezer Yudkowsky's Harry Potter and the Methods of Rationality and anything related to it. With two chapters recently the previous thread has very quickly reached 500 comments. The latest chapter as of 17th March 2012 is Ch. 79.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author's Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight, nine, ten.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: