Scenario 1) is very easy to acheive. You just use this option.
For scenario 2) the drop down control does not support conditional logic to change the configuration based on the number of options. While not the most elegant, you can consider just having two cells side by side with visible when conditions. When there is only 1 result for your source, display the control as is. Else, display the version which uses the placeholder above. Depending on your source data, this may or may not be complicated.