1) 766 is the minimum privilege you can use.
757 denies write permission of the user / group (see screen shot 1)
766 allows write for all users (see screen shot 2)
2) it looks like you renamed the file to artists_Inserttest.php, but the form action is still set to post to artists_Insert.php:
<form action="artists_Insert.php"
change that to:
<form action="artists_Inserttesst.php"