Not too sure about 1 but I'll try 2 and 3 :
When you deploy flaps and slats then the aerofoil coefficient of lift increases. Thus to maintain the same amount of total lift given the same airspeed then you will need a lower angle of attack.
If I remember correctly, to get a rough estimate of density altitude work out the temperature deviation from ISA and multiply this by 120 and then add/subtract this to pressure altitude.
eg. PA 4000'
Temp +27 (=ISA +20)
DA = 6400 (4000+(20 x 120))