There are a few characters that have a modified Leadership with bonuses for the characters that use the extra action.
For these benefits to activate, do they necessarily have to be the last action of the turn?

If at least two actions are given to Atlantis characters that turn, but the last two to take an action that turn do not, does it matter?

I can't think of any other examples off the top of my head...

Unless otherwise specified, they do not have to be the last action of the turn. You can assign them as you see fit. As always, make sure you make it clear as you do it.

Also note that you cannot gain more than one action from the normal use of any Leaderships, but than that limit only applies to "normal" use, not "special" ones : if you use any SP allowing to "use Leadership or to gain blablabla", and choose to gain blablabla instead of the normal use, it won't count toward your limit fro the turn.
In other word, any action gained from Moloch the Mystic would prevent you from gaining more action by using Leadership the normal way (and you would be able to use Willpower while using THAT action), but Undersea Ruler will allow you to gain another action with another Leadership of you choose to gain 2 actions with it (knowing that those 2 actions must be givent to caharacters with the Atlantis keyword, but that any third action gained with a normal use of Leadership won't have that limitation).
To summarize, if you play all three Moloch, Attuma and Namor, and if they all roll rightfor Leadership, you can choose to gain 5 extra actions for the turn : 4 must be assigned to characters with Atlantis keyword, the last one can be assigned to any character and will allow that character to use Willpower while resolving it.
Insane, neh ?