These questions are next to impossible to answer. The DUAL_EC_DRBG algorithm is widely agreed to be backdoored (even the author of this article seems to agree),

These questions are next to impossible to answer. The DUAL_EC_DRBG algorithm is widely agreed to be backdoored (even the author of this article seems to agree), but nobody has actually proven that an attack exists. Cryptography is a very complex field, Telegram has made many bizarre design decisions which make it difficult to trust them despite the fact that their encryption has not been publicly broken recently.