1. After pg_start_backup() and till pg_stop_backup() VACUUM is denied (e.g. autovacuum is turned off), so the new data is always appended to data files, is never written at their middle.
This is not the case. Autovacuum continues to run during the backup.
This allows to archive the data directory using any copying tools (rsync, tar, cp etc.). If you forget to call pg_stop_backup() by accident, data files will grow forever. So pg_start_backup() switches the database to "append-only mode" which is safe to backup without stopping (data files temporarily become append-only, WAL logs are append-only always).
No, it doesn't work that way. I don't know why appending would be any safer than normal updates would be anyway. WAL replay fixes up any problems that might arise.
2. After pg_start_backup() and till pg_stop_backup() full_page_writes is forced to be ON.
Effectively yes, this is documented in one of your links above (and is one of the reasons vacuuming during the backup is not a problem)
