Emma this in NOT about an epic battle between you and your ex. It's about your DS(s). Calm, cool, rational. Supply data. As I mentioned I am a former GAL although in different circumstances. I can tell you a parent calmly presenting detailed recommendations from qualified experts carried a lot of weight.
You are not trying to prove your ex screwed up - you are trying to determine what your DS needs. You have the basics in the NP report - these other reports can provide more specifics. Unless your ex provides similar documentation showing a different set of recommendations the question will likely become how to best meet the needs your experts have outlined. I am not familiar with a large GAL organization but if they have in house experts that's fine - they can review what you provide and either agree or make other recommendations. It's all data. I think this will go a long way to get GAL "on your side".
If your ex wants the kids at a religious school the burden will be on him to demonstrate that he has a school that can provide the detailed program that the experts have outlined. You will say that FAPE/IDEA will (should) ensure that the school district will provide an appropriate program. Whether he is placed at a public school or at a religious school where they pay the tuition the district should be overseeing his program to ensure his needs are being met. Wherever he is placed your DS needs to have his progress measured and monitored. You will argue that you are not anti religious school your ex is choosing (even if in reality you are...) you just want to make sure that the kids' needs are being met. Cool, calm, rational.
Doing this will hopefully also show the GAL that his/her recommendations should include you being allowed to be involved in discussions about schooling even if your ex has final decision making authority if you disagree. (Of course this means he would be given similar rights with medical discussions...) Ideally he will recommend a change in this decision making structure but right now you just need to focus on how to work within it to get your DS's what they need.
I would thank the ex for the invite and attend the open house. Ask open ended questions and see what they can offer. Do not appear confrontational or obstructionist. Just see what they offer. It could be a compromise, or at least better than what he would have if he stayed at current school.
From your posts I am guessing you are located in a specific huge school district where the 12-1-1 designation in an IEP opens the door to OOD placement.Totally different from what I have worked with so take my input with a grain of salt. Use what may help and ignore what doesn't.
I'm rooting for you! Hang in there.
