Are Data Pad values part of the secured content in GLP and are changes to the Data Pad logged to the Audit Log?

Yes. Changes made in the Data Pad are logged in the GLP Audit Log and do count as changes to the GLP Document. 

Any changes to cell selection in the Data Pad (i.e. selecting another cell or clicking in the current cell to make it active) will be logged in the Audit Log as a "Data Pad Set Cell" Modification Entry. This change forces the document to be saved with a different name. The values themselves are not logged, but the action of a change being made to the Data Pad is logged.