After validation, will I get errors again in future?
No! But that depends on you. Your website will remain valid until you will make any invalid change to the code or add new content. (But don't worry, once i have fixed your website I will fix it again with great discount that time)
Can I trust you with login credentials?
Yes, I have worked with so many clients and they trust me! As you know, these credentials will be required obviously to fix the website.
What if I am not confortable providing Login credentials?
Don't worry we will try to come with another solution. You could either create a temporary user for me (could be deleted after the work is done) or you could send me your website files. I will fix and send them back. (mostly in simple HTML websites)