In case of a drone strike with an airplane below 400 feet, who is at fault?
The FAA has some rules pending about this, you can read them here but for drones under 55 LBS one of the proposed rules is, 


  Must yield right-of-way to other aircraft, manned or unmanned.


and since you bring up class E it should be noted that 


  Operations in Class B, C, D and E airspace are
  allowed with the required ATC permission.


if/when this goes through it looks like its the drone operators fault since the plane had the right of way 

According to the FAA's website in relation to model aircraft they state that, 


  (4) the aircraft is operated in a manner that does not interfere with and gives way to any manned aircraft;


this is pretty much a blanket statement that maned aircraft have the right of way but the FAA handles most things on a case by cases basis so in the end of the day it could go either way.