BlueSwann the little girl can be an untiended offshot of ivala or her power. something acting outside her control.
and i don't think her "mommy" is ivala. ad this point this 'mommy' can be hester herself. after all xestris, a tower walker and demon hybrid it's the thing closer to 'mommy' we meet. and don't think she was a "gathering intelligence construct" made by ivala's captor, not because she doesn't want analyzed, but because she don't seek any useful information.
for why ivala want ask (intermediary person) about simon character? in the interlude between fourth and fifth chapter ivala said to tanurak to have little information of the world, only the hope the plan she set in motion long ago will bear the fruit she hope for. only he breaching her tower obfuscation allow to glimpse the world. and the little girl vanish between that chapter. so she can "reasorb" that fragment of her and learn about simon. (i don't think it's the explanation, but it's fit what we know)
i think was ivala to allow (or facilitate) riala find the two shards. and thus put simon on the track to save her. she obviously meddle with carina to push her in his arms. after that she stop any direct intervention, because she wasn't able to do more or because will leave her open to other (mithyn can visit ivala for few moment before anyone will detect her)
for the sip... if the little girl is a product/byproduct of divine magic a gain little residue of pure divine power can make her more powerful or 'persistent' so she can alter reality more, and when the effect of her works will be manifest it will be more benefical (or less benefical is she don't work for ivala, but for her captor) for simon.
it come me now... but we have alwasy speculate the "frustrated" create by the synod was about the church of ivala... but if it was because someone in the church was speaking about "the ivala will" when actualy be held ivala captive and harvesting her power?