1. The SK has been referred to already by Śabara and Kumārila and might have been commented on together with the Pūrvamīmāṃsāsūtra (and perhaps also with the Vedāntasūtra).
2. The SK was not at all important for Śabara and Kumārila.
3. The SK as we have it might be the one referred to by Śabara and Kumārila, given that the topics it deals with (i.e., ritual details) corresponds with what one would have expected.

However:

• At a certain point and in a certain area, the SK acquires a much more important role.
• Śaṅkara, Rāmānuja, Vedānta Deśika, etc., evoke the SK not for ritual details, but as the part of the whole mīmāṃsāśāstra which deals with deities or with their adoration and is correspondingly called devatākāṇḍa or upāsanakāṇḍa.

Is the SK really what lies behind the “new” devatākāṇḍa? Or has one reused the name of a less known part of the mīmāṃsāśāstra to introduce the topic of God in it?

On the SK, read this post.