The resurrection stone wouldn't do any good, and Beedle the Bard already told why in his tale of the Three Brothers. And Heromione already explained why you shouldn't abuse the time-turner hoping to prevent people's deaths. In the case of Buckbeak, Hermione did so by Dumbledore's order, but remember, unlike Hermione, Dumbledore already knew Buckbeak had gone missing, and may even have seen future Harry and Hermione run off with him, but distracted the attention from Fudge and McNair... a possibility.... So technically the time line WAS already altered, and as such no damage done... As a matter of fact, since future Hermione and future Harry did save the life of past Harry, there would have been a paradox if they did NOT go into the past.... Also note that very little people knew about this event. Cedric was brought back to Hogwarts dead for all the students of Hogwarts, Beauxbatons and Durmstrang to see, not to mention representatives of the ministry and the Daily Prophet... that would mess things up. In Snape's case... If Snape never died Harry wouldn't have known Snape's part of the tale, but also not that it was Harry's destiny to die to save the world from Voldemort. Voldemort would have won, that way. I cannot really speak for Fred, but I know that Sirius did not want people to meddle with time on his behalf... AGAIN (he was saved a Dementor's kiss before that way). Sirius was ready to die when he came to Harry's defense in the department of Mysteries. Trying to bring him back would have caused chaos, most likely in the Death Eater's benefit.
Overuse of the time turner can cause by far more damage than death can cause. Like Dumbledore said: To a well organized mind, death is but the next adventure.
The fact that there are things worse than death and that you have to accept your own mortality, is one of the biggest core messages behind Harry Potter. Knowing that, it was only natural people close to Harry would die... Harry would have to learn to accept death as a part of life, and be that way stronger and smarter than Voldemort who could never accept it....
Don't be like Voldemort, folks...
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