Yes, users can just buy the K3 mod separately from Aspire online store, here is the link of your reference: https://online.aspirecig.com/aspire-k3-battery-p-266.html
The glass tube of K3 tank is fixed to the top hardware, which is not changeable, if the glass tube is broken, users can buy the K3 tank separately from online store, here is the link for your reference: https://online.aspirecig.com/aspire-k3-tank-clearomizer-p-191.html
K3 tank is not airflow adjustable, the airflow is fixed
Can I use Nautilus BVC 1.6ohm coil or Nautilus 2 0.7ohm coil or Nautilus 2S 0.4ohm coil on the K3 mod?
No, as the output wattage of K3 mod may not suitable for these coils
We suggest that the wall adaptor specification is 5V/1A