Hi, Chris:
What logic would you base your selection off of? Would the user click a button next to that row? If you had a "copy" button in your repeating table, you could just have the user click it to set values in your main data source, no qRules required for that. I'm attaching a simple sample form. To use the sample, right click the link in the header. Save the file locally. Right click the file that you saved locally, and select "Design" to open in design mode. You can then preview the form.
As to question #3, I think what you are saying is that you want to submit back to your original SharePoint list? If so, qRules has the "SubmitToSharePointList" command that can do this.