I can see this being Donna all over again.

The doctor, having lost the battle and knowing how River's life ends, and how it affects Amy/Rory, will willingly change something that means River Song never exists as she is now, instead being a shadow of what she became, as she lives out her life as Melody Pond.

Amy and Rory would raise Melody in a new timeline that wipes out everything that happens from then on, which obviously affects earlier plot stories, and means the Doctor and River never have their time together.

There'd be no Silence in the Library because Melody would never grow up to be River, etc.

Or could this not happen because it'd affect his own personal timeline. Or would he just not do it because he promised River in SITL that he wouldn't meddle with timelines to save her, etc?

I just have this horrible feeling that the finale will involve Rory and Amy and Melody being back in Leadworth, leading normal lives, and River ceasing to exist, which would be as heartbreaking as Donna being reverted to her old self.

I hope not! I hope River is a recurring character for years and when Alex Kingston doesn't want to do it anymore she can "unregenerate" into someone else. To be honest, I'd be a bit annoyed if they did what you are describing. We've been waiting three years to find out who she is with them bigging up her backstory and a love affair and stuff, so if they got rid of her by the end of this series it would be a bit rubbish!

It would be worse, as it would completely undermine most of the events of the last two series, not to mention the related events in Series 4. At least when Donna lost her memory, the deeds she did still shone brightly.

Besides, River seems to have grown up into a relatively well-adjusted individual for being kidnapped and turned into a military killing machine, hasn't she?

What happens? Reapers. Watch Father's Day, it clearly shows the consequences of going back into your own timeline.