You can field up to three per character in the first example, and up to six in total in the second example. Combined Arms composition rules only restrict units with no other restriction on them. If a unit has any form of restriction, that takes precedence.

Note, however, that 1+ is not a restriction, it’s a minimum requirement.

No. If a unit is in a position to control two or more such objects, you must choose one for it to control.

No. To be more than double the enemy’s Unit Strength, the enemy has to have some models in the quarter. If they have none and you do, you gain the VP bonus for that achievement instead.