When using a sampler2DArray, will texture lookups perform linear interpolation between texture layers, i.e. if I use vec3(0,0,0.5) will that return a combination of layers 0 and 1?
No. That’s why it’s not a 3D texture.
The reason an array texture is compelling for this usage is because it does not merge mipmaps in the depth axis, so layers do not bleed into each other at lower resolution mip levels.