A discussion was held in the other aforementioned location, and the resultant information (provided by RoryA on Jun 20th, 2016) is as follows:

SELECT * INTO and INSERT INTO will only work when the destination workbook is closed whereas I am attempting to import data into an open workbook.

I am already importing the data using ADO CopyFromRecordSet command that works with an open destination workbook, but I keep randomly encountering problems with Excel appearing to either not release all of the memory after the code has finished or there is a memory leak somewhere because repeated executions of the code will cause Excel to eventually stop being able to even launch a simple UserForm stating that there insufficient storage to do so.

Since there are known memory leak issues with using ADO on open workbooks, I will have to consider what to do: change the approach of how the code and import should operate to maybe support using a closed intermediary workbook, open each source file and then obtain the data without using ADO, or live with the memory issue until everything gets ported over to something more capable of handling the massive amount of data.