No you aren't "over reacting".

Perhaps you could look again at the paperwork given to you about the trip and if that, coupled with the information you were given at the open evening, doesn't give explicit information about the Abernethy Trust and the planned "time out" sessions then the school has been dishonest (probably not intentionally) about the programme.

I've been a headteacher of a faith school (when I was a Christian) and used residential venues run by Christian organisations but I always made sure that all the activities were well known in advance. My residential trips were too full of other things to have had space for preaching but even if RE/preaching sessions had been on the itinerary, the parents would have known about it before the trip.

Re: your own child, I wouldn't make a big fuss about it after the event but it would be worth raising it with the school as something they should consider much more carefully for next year's residential trip.