2. Can E10K both power headphone and line out at the same time?

A: Yes, of course! But there are some information you need to pay attention.First ,you can't connect headphone to power out and connect another headphone to line out , that is to say both of two ends can't connect headphones at the same time. You can try to connect headphone to power out and connect the HI-FI to line out.
Second, there will be some influence on the headphone if you connect the HI-FI at the same time, if you want to reduce the influence to minimum, you need to make sure that the HI-FI's impedance is high enough, so that you will not get distorted or sound decrease, thanks.