Section 2: Second Rhetorical Question Summary

Get out the microscope, because we’re going through this poem line-by-line.

What could have made her peaceful with a mind
That nobleness made simple as a fire,

The beginning of the second question says, in effect, "She couldn't have turned out any other way, so what are you getting your undies in a bunch about!"
It's a rhetorical question, so the poet is talking to himself.
Remember how we said Yeats was conservative? He's big into aristocratic values like "nobleness," manners, and refinement.
Maud Gonne is not a simple person or commoner. She has a noble personality, like a medieval lord...or an ancient Greek warrior.
Yeats equates nobility with "simplicity" of mind, as if Gonne's emotions were not all muddled up and conflicted. Remember that famous "noble" characters, like Achilles from the Iliad, are often defined by a single, purified emotion – in Achilles' case, "rage."
Maud Gonne's state of mind is captured by the word "fire," as in "fiery." She can't be peaceful with a mind of fire.

With beauty like a tightened bow, a kind
That is not natural in an age like this,

Yeats moves from discussing Gonne's mind to discussing her appearance. Her beauty, like her emotions, is simple. He compares her appearance to a "tightened bow" and says that her kind of beauty is old-school; it belongs to another age.
The string on a bow (as in bows and arrows) must be kept very tight so that it can shoot arrows far. In Homer's ancient Greek work the Odyssey, Odysseus's bow is so tight that only he can string it. He's super strong.
A tightened bow looks graceful and noble, but it contains an enormous amount of power within the tension of that single string. Maud Gonne's beauty is the same way: graceful, but containing a deep inner tension that only adds to the allure.
The bow is often associated with ancient Greece, and because we know how much Yeats like dropping references to the good old Greeks, we can say pretty confidently that the "age" in which Gonne's beauty would seem most natural would be oh, say, a couple thousand years ago on the Mediterranean coast.
OK, we'll bring out the hammer and hit you with it: she looks like a character from one of Homer's epics. (Homer wrote both the Iliad and theOdyssey).

and the

Being high and solitary and most stern?

There he goes with more description of Gonne's noble beauty.
Doesn't this line make her sound like some kind of royalty, a queen or princess?
She is "high," like someone haughty or of "high" birth.
She is a "solitary" person, the kind of person who is confident in her own worth and does not need to be validated by others. She's like, special. Also, her beauty has no comparison; it stands alone.
Finally, she is "stern," or unyielding. Not just stern, but "most stern." Nobody's sterner than her! Actually "most" just means "really" or "very" here.
She's like that teacher you used to have who could reduce you to a blubbering idiot with just one "stern" glance.
She's not someone to mess with; she can hold her own.