Q1 : Is it correct ?
Yes. 6.4.3, last sentence: Mortars have no restrictions on firing over the heads of friendly troops.
Q2 : If correct, where to put the PDF marker for the 60mm mortar ?
one pointing from card A to card B, then another from card B to card C.
Q3 : Is it mandatory to have the CO HQ and 60mm mortar on the same card to fire with the 60mm mortar ?
No, because in this case the mortar can see and is firing Direct Lay (6.7.4). He doesn't need a spotter.
Q4 : Is it correct ?
Q5 : Is it correct ?
Yes. Per targeting priorities the LMG is required to open fire on B rather than on A (B is closer).