If he can go back to the same memory twice why can't he go back and do basically the same thing with something different to change the future. He could have gone back to the blackout during the filming of a kiddy porn and addressed the problem that came about as the result of what he did last time.
Except that after the explosives incident he had neither the journals nor photos to use to bring him back as the journal writing started after that blackout. Though he couldn't have rectified the brother's abuse then after the "jail bird" reality and thus saved a lot of grief...
And considering he can only go back in time by reading journals or looking at photos of events during his blackouts, assuming this is all based on him remembering things like they're actually happening again, how does using some old home movie of his mother's pregnancy and/or birth of him bring him back to that moment at such an age our brains don't develop enough to store such memories?
Exactly. Evans can go back where he DOESN'T have memories of the events. Thus, he can get back to the point where baby Evan was in the womb.
Actually the human brain is the first thing to develop and is essentially fully formed in the first month after conception, though it continues to grow. Given how far along his mother is shown to be pregnant, he could go back to that point.
It's also worth noting that throughout the film, his father's use of polaroids and his use of a diary, the mechanics of their form of time travel seems to be more based around visualisation of where they want to go, than actual memory itself. The video also worked because of this.
The theatrical ending is much better than the one in the director's cut. And here's why. In the theatrical cut, Evans goes back to when he first met Kayleigh and creeps her out so that she'll never be near him again. As a result, Kayleigh and her brother go to live with their mother. As a result, the awful childhood memories involving Evans and them never happened. However, Evans probably became friends with Lenny still, which explains why the two are still friends and sharing a dorm room at college. Now, let's think about the director's cut. Evans goes back to when he was in his mother's womb, and asphyxiates himself with the umbilical cord. As a result, he was never fully born, and therefore any and all interactions he has lived never happened. That includes Kayleigh and Tommy. This means that Kayleigh was never creeped out by Evans that she will leave with her mother. So, she stays with her father... and thus, the child pronography event still could have happened. Along with all the emotional scarring caused by it to Tommy. Maybe this saved them some of the more scarring childhood experiences (if Evans was the one to become friends with Lenny first, and introduced him to Kayleigh and Tommy, then with Evans never born, Lenny maybe has never met them, and thus all incidents with Lenny involved in some way never happened. It still doubles as Fridge Horror when you realize that the child pornography event might has big odds to have happened still.
You've got it backwards. It's not that Kayleigh only left because she was creeped out by Evan. She was going to leave anyway—what happened was in the original timeline, she stayed because she liked Evan and wanted to be near him. If there's no Evan, there's no reason for her to stay.