How is Seth's older self able to escape if his legs have been cut off? Shouldn't the timeline change?
It doesn't work that way. Anything you do to a looper's younger self does affect the looper's older self, but not until you do it.
No, it doesn't make a whole lot of sense. But it is what it is. And at the end of the day, the time travel only exists to support the needs of the plot. Best not to worry about it, or in other words, "Don't think so much."