As others have said, subjects can change, especially “interesting” more niche ones, as they’d typically just have one teacher who would take those classes - eg Astronomy, Latin - if that staff member leaves then they may not get someone else in as it may have been a bonus they had someone who doubled up as a science teacher and an astronomy teacher, for example.
Or one of the DT teachers leaves and they can’t get a replacement in on time.
Most schools will list their KS4 subjects on their websites - it’s usually easy enough to see what subjects they teach?
Regarding your third point - usually the students who end up taking more GCSEs are the much more able students who can choose to take gcse statistics, and / or further maths - these are often managed by eg the top set teaching at a faster pace (which is fine for the most able students) and covering a lot more material. At our school they weren’t compulsory even if you were in top set maths.
And again, more able students will often do three separate sciences rather than two combined qualifications - that adds another one on but you’re covering just a bit more material and it’s all timetabled.
So the more able students can readily have 3 extra GCSEs - stats, FM and an extra science qualification - and it’s more about covering more material in class and perhaps one extra after school lesson weekly. It’s not usually people also taking extra GCSE’s in history, French and computing on top of all the other ones people are doing.
I do think the “is RS compulsory” is really important, because children tend to prefer to have more choice over their options, and they might have to pick just two or three out of a whole massive list of history, geog, Spanish, French, German, computing, DT, food tech, business, music, drama, art, media studies, gcse PE. If your child likes a lot of these but eg not RS, it’s deeply annoying to have a compulsory slot taken up by it.