Short answers

Technically, yes — there is no end-to-end encryption yet. Stated on the front page and the security page because it should decide whether you use this at all.

Nothing. Not sold, profiled, shared with a broker, or used to train anything. The privacy page lists what is stored.

One person, on one rented server. No company, no investor.

The service stops with it. Blackbird is free and carries no guarantee of availability, so it should not be the only place something important lives.

Blackbird checks on launch, shows what changed, verifies the download against the server's fingerprint, and installs it.

Sign in the way you signed up — email and a one-time code. Add a password later from Settings → Account, or an email to a username account the same way.

No. It is optional, only matches numbers already on your phone against existing accounts, and keeps nothing that does not match.

From inside the app. Your messages go with it. A copy can survive briefly in a backup before that backup rotates out.

Publishing there requires a verified developer account — identity documents and a public address. Distributing directly avoids that. See the install page.

No, and none in progress.