A mind at peace with all below, (line 17)
The calmness of her face makes the speaker assume that she's "at peace" with everyone. There's nothing to make her anxious or worried.
A heart whose love is innocent! (line 18)
In the final line of the poem, the speaker reassures us (in case we were suspicious) that the woman is "innocent." She's not going to allow him to seduce her, or do anything else of that nature, so stop thinking that.