The errata now says that you don't gain the Basic Technique from the Discipline you choose as part of the Wyrdling mutation.

However Basic Techniques can't normally be purchased on their own, so can you even select them as one of the two Techniques that Wyrdling does give you?

Unless it says otherwise (and it doesn't) you can, but it will count as one of the two 'free' powers it grants you.

What if you don't choose it as one of your starting Techniques, but you are an Astropath? Can you buy it later?

I would say if you are an Astropath you would have to wait until you could buy the Psychic Discipline talent at rank 4, and chose that discipline.