You can look into this example(the same is available in your local copy of our WPF demos).
The change you have to make is to get the GridViewCell from the original source rather than the GridViewRow. You can then use the cell to update anything you want.
As to the first part of your response, I am not sure I get what you are trying to achieve here. Can you provide a more detailed expanation?
Thank you in advance!