Where is shikantaza taught the way you talk about? Is it a specific teacher, temple or lineage of Soto Zen? Or more of a philosophical branch?
There were clear instructions quoted and referred to here, and there are many more available. For instance, from the Soto church's official website:
"Do not concentrate on any particular object or control your thought. When you maintain a proper posture and your breathing settles down, your mind will naturally become tranquil.
When various thoughts arise in your mind, do not become caught up by them or struggle with them; neither pursue nor try to escape from them. Just leave thoughts alone, allowing them to come up and go away freely. The essential thing in doing zazen is to awaken (kakusoku) from distraction and dullness, and return to the right posture moment by moment."
Before and after this it gives all the ritual and physical acts one should be aware of, pictures included. What else could be there to it? Gives the same insturctions one finds everywhere. That's why I ask where all those other extras you mention come from.
What you write is correct for the general instruction for public. But still one needs meticulous explanation from the teacher, since in those words are dangers of ordinary understanding. What I wrote comes from different sources.
First as for the purification etc. it is in instructions and sodo practice, but could be done individually as well... those are manuals which are not translated. Commentaries as well.
For the second - ST instructions come from following lineages: Bokusan, Sotan, Kodo; then from Tokuo line and 8 teachers of combined lines ending with Sogaku, plus very particular line of Tenkei, down to Roryu and his successors. Plus one line of Eko, and his successors, the last whom I knew died almost 20 years ago, and I have no idea who inherited from him.
Then very very intense and powerful instructions to ST from Tanzan of Saijo... they are in part written by himself. Saijo is guardian of those. The lineages are not always in agreement, the example is of Tanzan and Sogaku.. Tanzan had a lot of subsequent instructions on breathing ST, which gave fruition of great JINZU, I think it is called siddhi in sanskrit. Anyway he had a lot of supernatural powers which is somehow connected to Saijo from its beginning... Sogaku did not say it is entirely wrong, but he said that it was really dangerous for ones life and health if it goes wrong. Actually Sogaku way was how to say double. The external conditions were wrathful, however overall conditions were totally relaxed. So for outside public he was known as ''devil'' it is lineage I knew best probably, and I knew 4 of his direct heirs and also number of his disciples. And there was some
Blessing of the lineage is required by sodos and all lineages, as I said it is practiced on ones own, since it comes from the very particular lineage of individual practitioner, often includes prayers to personal protection diety as well.
Plus there is now number of restricted materials published in Tohoku by the group of senior priests called Tekizen, which includes several temples of different importance. Tekizen mostly focuses on saving vast heritage of Sugimoto Roshi, but is today very popular among senior practitioners since includes many instructions on esoteric parts, and also on specific retreats like 100 day retreat based on the view of three kayas, and the path of Rishubun, secret part of Prajnaparamita, it is for those who completed their required training. This is one of the most extraordinary ways and I knew some priests and teachers very skilled in it.
One should of course mention great Toin, I met one Roshi who was student in his lineage and inherited Kodo, which was from the other edge. This one was great great master of ST with 1 or 2 disciples but I do not think that he was human
If you ask about academics, yes some do intense research on it, but they should be part of the inner circle.
But as you correctly pointed all this comes from specific teachers, temples and lineages of Soto Zen. Moreover what is very very interesting even those supportive or extraordinary practices, both for beginners (purification + blessing etc.) and advanced (Rishubun etc.) though are explained from common ground, but more interestingly they are presented in specific way of shikan taza as well.
However for the uneducated, looking from outside they may seem to have no relation with ST or have some exotic appearance. That is not correct since oldest kirigami we have are dated back to the XIV century, which is very early date for soto in japan. I am afraid that all these will be dropped or lost in the West. Including many ST instructions.
If there are misspellings, sorry...