Why didn't Moses accept Pharaoh's compromises?
Moses has clear "face-to-face" order from God what to tell Pharaoh - any compromise in this would deliberately bring Moses outside of God's will.
On the other hand it is it is interesting to see Moses to give Pharaoh several times the benefit of the doubt - Pharaoh promised to let people go, Moses prayed to take away the plague and then Pharaoh took his word back.
Would not it be "natural" (after the first bad experience) to revert the order - first let people go and then I will pray to end the plague? Can this be considered a compromise?
As for today In what instances should/should not church leaders accept compromise?
It is also too easy to say that church leader should not compromise on issues that would go against the word of God. In history the church divided many times on issues that seem to me unimportant but I believe that the people at that time considered them to be a clear order from God (women covering head, baptism of the spirit, ...) that cannot be compromised.
Also, Jesus compromised on issues that leaders of that time considered to be clearly forbidden by God, like touching leppers, meeting sinners, working on Sabath, ...
So I think it is always important to consider also guidance of the Holly Spirit and if there is not any other "higher law" that gives basis for compromise on things that seem to be clear God's orders (e.g. loving ones neighbour, passion more that sacrifices, love more than faith and knowledge, unity ...).