You could contact Khun Chatree of K&P directly for the amendment of the engagement letter. If you do not have contact information, Khun Pichanin will provide it to you.
The confidential agreement is unnecessary because the lawyer has to abide by the code of conduct, one of the obligations is to keep client's confidential and privileged. This engagement letter itself serves as a contract and has been reviewed and approved by BSJ legal and PS department.