This was a problem with the database character set for the connection.
in the Connections/njMySQL.php file, i added the following code at line 10 to correct the problem:
Note: if you use Dynamic Connections, you can select the character set.
as for you other question:
"The Primary Key field is set to auto-increment though, plus it's unique, so I can't just "create" a primary key value because it might already exist."
But this is an update, you are not creating a new record, you are updating an existing record., you need to tell the update which record to update by selecting the Primary key column and passing the ID using one of the form elements, otherwise it wont know which record to update. . .