Given that it measures both a solider’s possibility to make use of certain abilities, and also their resistance to panic, I’m wondering whether willpower should instead be called ‘control’.
You’ve got ‘control’ of abilities, and a ‘loss of control’ which leads to panic.
I kinda also like the term the ‘Heebie-Jeebie’ meter. 
I think Willpower does the best for encapsulating what all it does. Given that it encapsulates multiple things you could break out any one thing and say that X is a better name because of Y but ultimately I think they all circle back around to one’s Will.
Willpower works.
will·pow·er
ˈwilˌpou(ə)r
noun
control exerted to do something or restrain impulses.
Focus might be better
I think it works. It’s an abstraction for sanity, stamina, mental fortitude, and…well…will.
That said, I still think it’s not a good resource to draw equipment uses from.
grumble
I think given it’s use(both limiting the ability & equipment usage and panic threshold), it should be called “mental exhaustion” or “combat fatigue” and it should be a fillable meter, not reducible pool.
In that sense, making it a soft cap rather that a hard cap it is now(i.e. allowing certain abilities to still be performed after reaching the limit, albeit with higher and higher penalties) will make it more organic, compared to it’s current “mana pool” behaviour.
I was a bit worried it might seem weird to use willpower for overwatches and special abilities, but it actually makes sense.
To be able to hold your concentration and look for enemies to shoot when they enter your field of view, you need willpower…
To be able to excert yourself to shoot several times with your pistol and maintain the level of concentration and awareness it takes, you need willpower…
Willpower is self-restraint and the power to control ones actions, not only due to terror or fatigue. I think it encompasses it very nicely.