A manager can create a public broadcast channel and send the invite link to the employees. The manager can post topics in the channel. An employee can choose a message in the channel and select reply privately.
webxdc will only work one-way with the present API, unless the manager (or their bot) sends the file to each employee in a private message.
Addendum:
You could implement another, less expensive workaround. The survey host and the donors could participate in a single, regular private Delta Chat group. The survey host sends in a special webxdc that the donors would fill in. The webxdc comes bundled with a dedicated public key for the survey host. The webxdc as ran by the donors would encrypt their answers using public key cryptography in a way that only the survey host could decrypt, not other donors. You could either bundle a simple crypto library or use the subtle crypto API already implemented in most browsers: