a) If activating the main army general, can he pick up brigades under his subordinate army (rather than his own?)
No. 8.3, case 1 states that a General must be Active in order to place Brigades on his Command Display. Since the Subordinate isn't Active, he cannot receive recruits.
b) If no, can it still be done when an enemy army moves in an area before battle (pick up brigade into subordinate army).
Is there any case where this would be possible? or will I have to activate the subordinate army first to pick up that brigade?
Yes. 8.3, case 2 applies here, and the requirement is merely that a Friendly Army is in the Area and is eligible to receive those Brigades. It places no restriction on where those Brigades can be placed in the Army.
c) If subordinate army has a siege train, and main army is activated - does it still contribute to the bombard (even though siege train is under subordinate?)
Yes. 14.3 only requires that the Siege Train be present to contribute.
d) With turncoats, who chooses who turns - the loser/winner?
The winner. 10.6: "...the winner [...] may add [...] from those eliminated". Note that by 10.2, the loser selected which Brigades were lost, restricting the Turncoat choices.