The upper 2 rows contain 12 mercs max. There are more than 12 slots so you have more freedom in setting up your formation. The two rows represent how your men will be deployed in battle (facing upwards).
Will one be able to save and load different formations, making one’s own preset formations?
The tents are for your reserve. You can switch your men around freely and fight with less than 12 if you want to.
So there will be 17 reserve slots?
Thus will it be theoretically possible to have 18 mercs in your party but send for example only that one Swordmaster into the fight against a pack of direwolves to level him faster?