To understand the supreme being and the process of creation started by the supreme being, one has to read the Shakteya Agamas. All the Hindu deities including the Trimurthis Brahma, Vishnu and Rudra coalesce into Moola Prakriti(distinct from Prakriti/Purusha) after Pralaya. 

A detailed analysis of Saktheya Agamas is provided in the book attached in the following website. 

http://agasthyayug.blogspot.com/

SadaShivam is Guru and corresponding Devi is Moksha Shakthi. My inference is Sadashiva is Ishwara. 

        

> SA Poorveshamapi Guru Kalena Anavachedat - Patanjali YogaSutras
> Samadhi Pada 26
> 
> 
> Translation: Iswara is Guru (Preceptor) to the most ancient, and He surpasses Time, not being subject to Time.

The position of Sadashivam in the heirarchy is given below (refer chapter XVIII in the book)

[![enter image description here][1]][1]


  [1]: https://i.sstatic.net/Q4vEl.jpg

I am not sure about Shakshat Shiva. It could be Aham Sathyam(Aham of Moola Prakriti), mentioned in the book.