Yes when I was a JW I thought if the big A and the paradise is a reality then forbidding Xmas, our children having birthdays, worldly friends and blood transfusions makes sense, the end justifies the means. It did matter to me if it was correct however because if it's all nonsense then it means letting kids be screwed up by being isolated from normality at best and at worst dying from lack of medical treatment.
Now I tend to take the view that denying kids normal human friendship and fun is as you put it 'no way to treat someone'. If there is a God why would he do that? So in that sense I agree that often the best way to find out if a religion is 'right' and 'true' is if it works for human beings. No loving God would want us to spend a lifetime being miserable now for a beautiful future. This is a dark and medieval way of seeing the world. An omniscient being would not treat humans like this, he would know it wasn't necessary.
On top of that, the most important thing in the situation is how you treat the person/the practical results of pursuing a belief system.
Yes but in practical terms a belief system, a religion, entails people making rules that impact on your life. Don't have an abortion, a divorce, a blood transfusion, sex before marriage, a same-sex partner, a vote, an education etc. Beliefs impact on reality, whether they are true or not.