If I remember my three year old rumors correctly, Vader did appear in the climax of the initial cut of the movie. But the hallway fight was something the studio asked to be amped up in reshoots. Reshoots, that depending on who you believe either Edwards was not available for or not welcome.<p>The Lord of the Rings director jumped at the opportunity to see one of the world's most famous characters in action </p>www.independent.co.uk
The ending was also reworked to make the "Escape of the plans" sequence the ending rather than Jyn and Cassian on Skarif. Which necessitated new footage to make that sequence a clear ending.
So depending on your disposition as to the studio politics, either Edwards couldn't cut it and they brought in Gilroy to fix everything. Or, they discovered some late tweaks to improve the movie and they used a ringer to chip in when Edwards couldn't do it.
My guess would likely be that the truth is somewhere in the middle, as it most often is.