Thank you to Michael Witzel who pointed me to his edition and translation of The Katha Aranyaka 2.100 and was kind enough to translate the passage into english.
And thank you to Corinna Wessels-Mevissen who pointed me to Stella Kramrisch's The Presence of Shiva. I was pleasantly surprised to see most (all?) of Stella Kramrisch's books available for e-loan on archive.org.
If anyone has any articles on the development of the idea of Shiva. Particularly how vedic Rudra became Shiva. Could you forward them to me, or point out if there is anything on archive.org etc.