1) You should install the attached sample custom module for 5.2 version.
Just install the module using the
"Upload add-on" button, and you do not need to create any files manually.
2) If you upload/create the module's files manually, the you should create the following directories:
classes/XLite/Module/XCartDevs/ChangeSearchDefaults
classes/XLite/Module/XCartDevs/ChangeSearchDefaults/View/Form/Product/Search/Customer
And put the "Main.php" and "SimpleForm.php" files into respective directories.
3) As for the 3rd question: how did you edit and save the respective files, what file editor did you use?