I agree that the original plan was to kill April off. The entire season, she was in a downward spiral: drinking, partying, coming to work hungover, having sex with interns, not believing, etc. She even did an "Apology Tour" and told Jackson that she was happy that Harriet had him to look after her. It really seemed like they were foreshadowing either suicide or a drunk driving accident resulting in death (probably with April behind the wheel - could have been the original plan for 14x23).
If they didn't anticipate the fan backlash, they may have been ordered to change April's ending to a happier one. The ending they gave April...makes no sense at all. The timeline doesn't make sense and it seems like they literally pulled it together at the last minute. In their defense: Shonda has said that Derek and Mark needed to die when the actors left because neither character would leave their children behind. There's no way April would leave Harriet and Jackson would never let April take Harriet if April moved out of Seattle. So their options are basically limited to death or coming up with a reason that April stays in Seattle but is never seen. So, if they were forced to change to a happy exit, they're basically forced to come up with a reason that she stays in Seattle for Harriet but doesn't work at the hospital.