You can either show the results by the screen or send each result only to the corresponding person.
It is not necessary for all participants to install the app. An URL will be generated and the participants can access via any browser. You can share it via Whatsapp, Messenger, email, etc.
The secret is guaranteed. Even though a participant can open a different envelope, envelopes can only be opened once, so if this happens then another person would claim there is no envelope with his name and the process would have to be repeated.