Yes, updates work the same, I believe there might be a way to enable automatic updates too. And for flatpak I don't know what you mean with "manually update", usually you can just run "flatpak update" on the terminal and be fine, I don't know Mint enough to know if the software center updates them automatically too, look in the settings of it (if there are any). Hope this was helpful.
Probably around 800mb, but It has been long since I've used any desktop environment. Nowadays I prefer tiling window managers, they are much more lightweight and fast.
What bar is that?