Transformer modelling and impedance
Answering part 3, as secondary load increases from infinite Z, secondary current is drawn and that current induces a voltage in the primary. The polarity of induction is opposing the external primary energy source; given some tiny source impedance, the voltage across that source impedance is increasing, thus the primary current is increasing, 







simulate this circuit &ndash; Schematic created using CircuitLab

at the exact rate to provide the secondary power.