Thanks for the link. That sounds really interesting and it sounds like you did do a good job of thinking through what needed to be done to make a successful experience. No matter how well you prepared, there is always room for improvement. So look at the whole thing as a learning activity.
For the part about the religious songs being shared; that sounds like it was an open sharing session where the camp didn't really have any control over what the campers would sing. I would cover it in the constructive criticism letter and maybe the camp could give a reminder at the beginning of the sing along. I haven't been involved in girl scouts myself, but I know people who are and in some troups, when they do sing-alongs, they do sing religious songs. Some troups might think it's OK because no one in their troup complains when they do it.
For the camp in general part; I think the more communication the better, but you don't want to risk scaring the organizer of the camp by telling them more than they need to know. Try to think of what the organizers could have done that would have been helpful to your group. Was there anything they did that was really helpful, anything that was disruptive because of the way it was done, anything they tried to do for your group that would have worked if they had done it a little different? This might have been their first time working with a special needs group, so give them praise for whatever they tried to do and suggestions for next time (all without making them think "this group is too much trouble").
You can't control the camp, but you can do some things to help prepare your troup. Last year you didn't know what to expect, but this year you will have some experience to build on. Think of things that your troup might do to prepare. What about the team-building exercises was hard? Are there any things you can do during troup meetings that would help them over that hurdle? You might not be able to do the same activity, but anything similar or anything that helps build temas will give your girls a touchstone to make them more comfortable at the camp.
Good luck and thanks for sharing.