بسم الله الرحمن الرحيم
(1) According to Hanafi Fiqh it is makrooh to offer tahajjud salah in congregation with tada’i i.e. there are more than three muqtadis. It is even worse to call people by announcing for doing makrooh task.
(2) According to Hanafi Fiqh it is wajib to offer three rakah of witr with single salam. If someone offered witr salah in a way that he offered two rakah with one salam and then one rakah with another salam then as per Hanafi Fiqh this witr will not be valid. It is mandatory for him to offer three rakah with one salam, this is the preferred opinion. It is not lawful for a Hanafi Imam to lead witr salah with two salam.
Allah knows Best!
Darul Uloom Deoband