additionally, almost nothing in your uv sheet is perfectly straight. Anything that's on a slight angle/not pixel straight will have tons of stepping/aliasing in your bakes and textures.
yes and yes. I believe cut-out will have some flat overhead for sampling your alpha channel regardless of whether any pixels are actually 'cut'. Fabi_G has given a pretty definite answer.
I stopped to follow new nodes SD is adding lately so maybe it's a new one. But old "RT shadows" node I sometimes use needs only height and some manual sun position input. Not from a texture, just same vector for every pixel .