they're locked to a small set of pre-existing values (my account name, and email-address-only) rather than accepting free text.
This is the intended function.
There is a relevant risk that someone could abuse www.soscisurvey.de so send SPAM or fraud emails. Therefore, uless negotioated differently, you can only send emails under your email address or the generic sender address.
I've also added a colleague with full access, but her name doesn't appear as an option either
If she is working in the survey project, she can send emails with her name as sender.
If you want to use a project-specific address, you can create a SoSci Survey account with this address, grant the new account access to the survey project, and send emails from this account.