Perhaps I can help with number 1.
I think density altitude is calculated by multiplying the ISA deviation in Deg C. by 118 then that figure is added or subtracted to the aircrafts pressure altitude to get density altitude, which is only really a performance consideration.
The 4Deg/1000ft thing is TRUE altitude which is you actual altitude if you got out and measured it with a stick and not what the altimeter says( due to temp error). Basically i think it means that the atimeter will show an error of 4ft for every 1000ft of altitude AND degree deviation from ISA.
So at 10000ft if temp is -20 deg.
thats 4x10x15 (ISA @10000 is -5)
600ft. and its colder so you are 600ft lower than your altimeter reads!