Only one side of my 4iii power meter is pairing with the Garmin as well as only the tail light is pairing and not the headlight. How can I get both sides and both lights to pair?

I am having exactly the same issue with my 4iiii Dual Power meters and my Garmin 830

If you are pairing over ANT you only pair to the controlling side of the power meter. The controlling side then manages the communication with the other side.

You may need to make sure that the two sides of the power meter are paired together.

Have paired them in the 4iiii app so all good there. I then see just one ANT number which is what 4iiiii said would happen. But still am not seeing balance data which is confusing me. So now contemplating over a beer

That sounds like the correct configuration. Does the total power number look correct? Try doing some one sided pedaling and see what you get.