Can a Pro user use Passportal client credentials to perform work but NOT view them?
This is not possible.
Removing the permission Password: View from a users account permissions removes the users ability to access credentials in any way (except for the users My Vault credentials).
A 'hidden' password when used for a web portal will at some point in that interaction be in plain text in the browser code. Anyone with a base level coding knowledge, or a fairly technical understanding of things knows that there are easy ways to trap passwords. So if that person really wanted to know that password they could get it even though the password is 'hidden'.