(1) You should stop eating Sehri exactly five or six minutes before the correct time of Subh Sadiq (true dawn) according to a calendar of the city where you are staying. However it is better if you stop eating Sehri ten minutes before.
According to you, when azan is not called in right time then you should not make azan a criterion to stop eating Sehri, rather you should learn about the correct calendar and timing by consulting the local Ulama of the city where you are. Also keep your watch and alarm up to date. In-shaAllah, your fast shall be free from doubt.
(2) Azan should be called 1.5 or 2 minutes later after the correct time of sunset as a precautionary measure.
Allah knows Best!
Darul Uloom Deoband