How to get a WPF DataGrid cell to right align without making the selectable area on a new row tiny?

You could apply ElementStyle on the DataGridTextColumn targetted to TextBlock and right align that, it would work.

      <DataGridTextColumn Binding="{Binding Path=ImpaId}">
          <DataGridTextColumn.Header>
               <TextBlock  Style="{StaticResource
                                  DataGridHeader}">
                    Impa
               </TextBlock>
          </DataGridTextColumn.Header>
          <DataGridTextColumn.ElementStyle>
              <Style TargetType="{x:Type TextBlock}">
                  <Setter Property="HorizontalAlignment" Value="Right" />
              </Style>
          </DataGridTextColumn.ElementStyle>
      </DataGridTextColumn>