How does fluid pass through the glial limitans into the subarachnoid space?
Excellent question. This is an active area of research. 

The glia limitans defines the external most layer of brain parenchyma. Previously thought to primarily be a barrier against migration and growth of neural cells out of the brain parenchyma during development, recent work has characterized its involvement in regulating the immune environment of the CNS. A 2017 JCI paper demonstrated that the tight junctions of the glia limitans are inducible in mouse in response to inflammatory cues. You may find the associated editorial helpful. In addition to demonstrating a dynamic role for the barrier function, you can see that tight junctions are not the standard state, and the typical state allows passive diffusion as well as active transport of small molecules between the CSF and brain ISF, but the question is not fully resolved. 

Importantly, this is mouse data, and mouse data in particular cannot fully resolve questions about human astrocytes.