I don’t mean to be rude here, but these questions are un-answerable unless we know whether you “run hot” or “run cold,” and unless you know whether I “run hot” or “run cold” when I give you my opinion. For instance, I disagree with what my buddy @El Cascayal wrote above, when she said you only need a liner in July. I don’t disagree that she only needed a liner in July, but I did need a sleeping bag. I always carry a lightweight one, about 500g, and I have not walked a single camino when I did not use it. Sometimes I have been in albergues with people sleeping in their underwear with no covering, and I was in my bag. There is such a huge range of comfort levels!
Once you have walked a camino you will know what works for you, but until then I would go with your instinct. If you’re one of the ones who always brings a sweater to deal with the AC in summer time, you probably need one. If you run around in short sleeves in winter time, you probably don’t.
Gender and body weight (padding?) also have something to do with it.
It’s not an irreversible decision in most cases, since you can sleep in your rain jacket, put socks on your hands, etc., if you are cold, or send your sleeping bag ahead to Santiago if you find you just don’t need it.