Did Macon end up happily ever after with Muriel?
The ending certainly gives us the idea that Macon and Muriel will get back together. The turning point for Macon is when he injures his back in Paris, and Sarah comes to take care of him. The time he's spent with Muriel gives Macon an opportunity to compare his behavior and attitude when he's with Sarah to how he is when he's with Muriel. He realizes now that when he's with Sarah he is aloof and disconnected. The person he is with Sarah is "locked inside". Sarah offers him sleeping pills, and when he refuses the pills, he rejects the person he becomes when he's with Sarah.
Picking Muriel up in his taxicab indicates that he will start his life over again with Muriel. I think they have a good shot of living "happily ever after" since Macon has been through the worst and lowest parts of his life.