Here's my vote, as a person who walked in early August:
1) DO I NEED A SLEEPING BAG OR IS A LINER SUFFICIENT :?:
A liner is more than sufficient. Those refugios get HOT HOT HOT with all the people in them.
2) DO I NEED TO TAKE A FLEECE AND/OR LONG-SLEEVED SHIRT :?:
Personally, I would take a LIGHTWEIGHT fleece because even in summer, it can get chilly, especially if there is a rainstorm. Just take something featherweight that you can layer.
3) DO I NEED A RAIN JACKET/PONCHO :?:
Yes... I would take a lightweight poncho because it rains in Galicia... unless you're a duck and don't mind getting wet. Having said that, you may only see rain a few days out of the entire time, but for me, I hate being wet and chilled... and a poncho doesn't weigh much.
On the other hand, you can pick up long sleeve shirts and ponchos in Spain, probably in the free boxes in the alburgues...it's not a 3d world country... they have great shopping in the cities!
Whatever you decide, you'll be FINE! :::laughing:::
My last advice is this: If you look at something and you can't decide whether to take it or not.... LEAVE IT HOME. If you get there and need it, it will appear.
I'm going to start a thread about how the Camino supplies your needs... it will be fun to read and maybe will help give you confidence to leave the extra stuff home.