CO2 has carbon in it. Strip off the oxygen to breathe, the rest makes very strong building material.

Attach hydrogen, you get rocket fuel. Or plastic. Bind in some nitrogen, you can have proteins. There is probably silane lower down; send down a balloon to collect it, for silicon. Or all the way to the surface, to collect rocks.

Nuke power is totally practical there; no shielding needed, or worry about leaks; you just hang it a mile below your living-space balloons.

You can live there long term since there is gravity.

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 :)