I am in a little bit of a crunch here

I am a choral conductor at a large church in the Midwest.
During the month of September, I was asked to play 2-3 piano accompaniments for a large convention where a 200-piece chorus will sing. I will conduct one of the pieces, but I have also been asked to play the piano accompaniments for some of the pieces.
Here's where the 'crunch' begins...
Here are a few questions:
1. What are some effective ways to learn the accompaniments, that members use in here?
2. Do I try to memorize the piano score, or do I just read it?
Considering that my sight-reading level on the piano is probably average, or maybe slightly below average, I am quite worried, especially since practices will start in late-June.
Any advice would be greatly appreciated.
Thank you.