My suggested reading reference on Statistical Process Control is the book Statistical Quality Control, Grant/Leavenworth. There are several examples throughout the book offering visual representations/solutions and explanations.
Please forgive me if I am not understanding your question completely. From your posts above, you are asking about how variation is measured. There are two kinds of variation: common cause and special cause.
Variation in a process is measured by the dispersion of your data point on and SPC chart. The dispersion is representative of common and special cause variation acting together or independently (the chart shows both). When only common cause is present, the data points will be within the control limits (sometime adjustable limits). When a special cause is present, this is noted by a point outside of the limits or detectable by shift, trends, or unusual patterns as detected within sigma regions.
With your example above, it appears that processes you want to measure are the Variability of the Data/Documentation Process or the Variability of the Corrective/Preventive Action Process (please correct me if I am wrong). Both types of data could be used here to measure the variation in the processes. For instance, you might use a Run Chart to see how long it takes on average for a Draft document to be reviewed. Perhaps you use a chart for different types of documentation (i.e. a procedure, instruction, process sheet, or test plan). How long does it take? Plot your data. Apply the SPC formulas and determine trial limits. Continue to plot data. What do you see? Common cause variation or did you notice a special cause data point here or there? Investigate and remove the special causes. For the Corrective Action process you might want to try something similar such as the time it takes to respond to the receipt of a CAR. How long was it before you heard back that the recipient got the CAR (not how long it took to answer)? Do the same. What do you see? Investigate, remove causes, and improve the process.
To be honest, I have had nice success in using Run Charts to understand how processes flow. I have used it to determine availability of space in a warehouse, average time per inspection, time to receive product on line, Customer Service calls answered, etc.. Folks here have used them to help them understand staffing levels, efficiencies, and of course, process improvements. No matter how crude, they visually represent the process, and once brought under control, help to predict outcomes.
I hope this helps.