We can all agree about the ruletext being vague:
Diplomacy is generally ineffective in combat and against creatures that intend to harm you or your allies in the immediate future.
Is not really a serious ruletext, and can be read as:
(1) As you take 1 minute to make your diplomacy check, you will have a hard time doing it during a fight or on someone who is trying to kill you.
Or can be read as:
(2) A diplomacy check require the target to listen to you and you can't perform it on people who really don't want to be your friend.
Both are coherent, and it is up to the GM to choose which interpretation to use.
With the interpretation (1) you can use your diplomacy during a fight, even if it will be hard. Fight in Pathfinder rarely lasts for 1 minute. The GM may rule that diplomacy don't always take exactly 1 minute and add a random factor, and make it fail automatically if you operate offensive actions toward the target or its friends.
With the interpretation (2) you can't use it in a fight, but you still can use it against hostile creatures that are not in a fight (for example because you tied them up or because they are in jail, or because they are Shelyn paladins who can't attack first).
Anyways there are quicker ways to improve one's attitude, like with the charm spells (like charm person) or the witch charm hex, and these work in a fight.