Bloody hell, you lot are precious aren't you.
"I can see advantages in Ds using re time to do homework or have his guitar lesson."
If he were to do homework then he would have to be supervised, or in the back of some other teachers class, not convinient and not cost effective for one on one supervision. Would you like him to go to isolation for this period, as thats how I can see it being workable.
The guitar lessons are not arranged around when it is convinient for your son but for the teacher or the peripatetic teacher. If neither of these are avaialable at this time then your son will be doing the above.
"It should be unlawful for schools to make it compulsory to continue studying it for GCSE."
But in fact the opposite is true, schools are required to provide some religious education.
So what they do is, make the students sit the full or half course GCSE so that they take it seriously and these lessons do not cause big behaviour problems. It also allows schools to hire and retain RE specialists and put it on the timetable for the whole school, rather than shoehorning it in somewhere else with non-specialsts. Also the latter shoehorn option has oft been criticised by OFSTED so having formal RE lessons just makes things clearer and easier for the school, and the majority of parents do want some form of religious and moral education.
Mumsnet is the only place where state schools and teachers are mistaken for customer service, you want bespoke service go private. You knew what the schools policy was when you joined, and if you didn't more fool you. As stated before, the dropping of RE will probably not result in the ability to take music. Nor is not getting a GCSE music at school a hinderence to progression to do it at higher levels.
Your just making a fuss, and the people backing you are the typical mumsnet fantasists and moaners, of course your precious children are more valuable than anyone elses in the school community, but only to you, remember that.