Did you know that you can set the URL and that credentials will be requested the first time Collect tries to connect? The credentials can then be entered once and they will be saved. I would set up a QR code with all of your desired admin settings locking everything down including every general setting and setting only a server. Between data collectors, I would delete the project (requires admin access) and then scan that QR code to start with a fresh project. The data collector will then be prompted to enter their credentials when they refresh Fill Blank Form (or go to Get Blank Form if you choose not to use the "match exactly" form update mode). Alternately, you can have a more lax QR code that doesn't lock things down through an admin password but still sets the server and hides server settings. That way data collectors can delete their own project when done. I think the introduction of a first launch screen will help with this because it will allow you to be in a state where all projects are deleted.
This really will depend on your workflow. You could have multiple projects that are each configured for a specific data collector. Note that username will be shown in the project listing. It sounds like your specific data collectors are likely to make mistakes so it's probably not a good option until we introduce a way to require credentials to switch into a profile.
You could have data collectors set up multiple profiles with the SAME credentials and download a different subset of forms into each. For example, if the same individual collects data about X on Monday and Wednesday and about Y on Tuesday and Thursday (e.g. clinic visits vs. home visits), it can be helpful to download the subset of forms about X in a project called X and the subset of forms about Y in a project called Y.