I do believe that pro se litigation is a substitute of "actions of any kind".  One of the references you deleted was

http://www1.law.nyu.edu/pubs/annualsurvey/documents/60%20N.Y.U.%20Ann.%20Surv.%20Am.%20L.%20453%20(2005).pdf.

There is a discussion of history of the bar and the self-representation in there.

Please source this statement "In the United States, self-representation is permitted in most instances" and show sources as to when it is not permitted.
The statement "In the United States, self-representation is permitted in most instances" is not sourced. Can you source that and write a precise statement as to when pro se litigation is not allowed?  I think you should divide that into civil and criminal.  I don't think it is necessary to discuss corporations.  Please list people and types of people or types of actions in which self-representation is not permitted.