With the oxygen thing, I think that things will generally resist being totally fundamentally changed. Modifying it is fine, like making a hole bigger. So, she could probably increase or decrease the amount of oxygen in an area, or mix in nitrogen along with the oxygen. Something like that.
I think sealing light is a lot like sealing eyes. With eyes, since they're often closed, when there's a temporary switch from open to closed, there's a moment where the shields are down and they can be sealed shut. So, if it's light out, she can't just seal off the light (though she could partially seal it), but if it's light out and something block out the sun or whatever, she would then have an opportunity to seal away the light.
Also, anything that is currently active will only resist things that go against it. I mean, technically things are always "active" in a sense. When the eyes are closed, they're "active" against being opened, but "inactive" to being closed. So when someone's eyes are open, they're mostly defenseless to having them kept open and things like that.
Hope that cleared things up a bit. I'll be posting a second character in a bit.
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DROP THE R AND BACK AWAY!