Also, there is a lot of protection from radiation, from the mass of atmosphere above you and the mass of the planet below you.

Is Mars' gravity too low for long-term habitation?

I know that we know that zero-G is a negative, but do we have any idea if it's a threshold that needs to be reached, a linear response to G-force that scales from untenable to fine, etc?

I would imagine that it's going to be a gradient. Just like any other form of exercise, the more effort you put in, the more positive effect. Humans would need to put in much less effort on Mars. Once we are spread across the solar system there will probably new exercise systems for 0 and .3 G environments.

I am also unsure about how it would affect the prenatal development as the way "layering" occurs is highly dependent on gravity.

The only way to really find out is to go there and live for a while :)