If I understand you correctly, you want the user to be able to enter a path both as "Root\OTA_Demo_TestSets" and as "Root\OTA_Demo_TestSets\OTA_TS_2". In the first case you want to list all test instances in the test sets in the OTA_Demo_TestSets folder, and in the second case you only want the test instances in OTA_TS_2.
If this is what you mean, you could first assume that the last item in the path is a folder. In that case you should get a TestSetFolder returned. If no folder is returned, try returning a folder where the path is the substring up till the last backslash and find a test set with a name that equals the last part of the string.
If none of these alternatives work you have to assume that the path input by the user is invalid.
