Praise be to Allah.
Yes, in your situation it is permissible to combine Maghrib and ‘Isha’ prayers. It is narrated in the Sunnah that the Prophet (peace and blessings of Allaah be upon him) used to join Maghrib and ‘Isha’ prayers because of rain. Muslim (705) narrated from Sa’eed ibn Jubayr that Ibn ‘Abbaas (may Allaah be pleased with him) said: The Messenger of Allaah (peace and blessings of Allaah be upon him) joined Zuhr and ‘Asr, and Maghrib and ‘Isha’ in Madeenah at times other than times of fear or rain. I said to Ibn ‘Abbaas: Why did he do that? He said: So that his ummah would not be faced with hardship.
The fact that Ibn ‘Abbaas (may Allaah be pleased with him) stated that it was not because of fear or rain indicates that these are two of the reasons for joining prayers.
Shaykh al-Islam [Ibn Taymiyah] said:
It is permissible to combine the two evening prayers because of rain, strong cold winds or muddy conditions and the like, and this is the more correct scholarly view. This is the view of the madhhabs of Ahmad, Maalik and others.
And he said: It is permissible to combine prayers in the case of severely muddy conditions or severe cold wind on a dark night, etc, even if rain is not actually falling, according to the more sound of the two scholarly opinions. That is better than praying in one's home, because not joining the prayers in the mosque and praying at home instead is an innovation that is contrary to the Sunnah. The Sunnah is to offer the five daily prayers in the mosque in congregation, and that is better than praying at home, according to the consensus of the Muslims. Joining the prayers in the mosque is better than praying them separately at home according to the consensus of those imams who regard joining the prayers as permissible, such as Maalik and al-Shaafa’i. End quote.
Majmoo’ al-Fataawa, 24/30
In your situation, the reason for joining the prayers is undoubtedly more pressing than rain.
Joining the prayers in this case applies to those who are required to pray in congregation. As for sick people who pray at home, or women who pray at home, it is not permissible for them to join their prayers.
Shaykh Ibn ‘Uthaymeen said in al-Sharh al-Mumti’, 4/288
But if a person prays at home because of sickness and does not attend the mosque, then it is not permissible for him to join the prayers, because that will not benefit him. Similarly a woman is not permitted to join her prayers because of rain, because she will not benefit from doing so, as she is not one of those who are required to pray in congregation. End quote.
See also the answer to question no. 31172.
With regard to his giving the adhaan for ‘Isha’ at the time for ‘Isha’, there is nothing wrong with that, so that those who are praying at home, such as women and the sick, will know that the time for ‘Isha’ prayer has begun.
Your describing the imam of your mosque as not having much knowledge of fiqh because he joins Maghrib and ‘Isha’ would seem to be a mistake, as you now know that this action of his is correct, based on the evidence noted above.
You should have been easygoing and not hastened to denounce something until you knew whether it was wrong. You should not attribute ignorance to your brother without proof.
The fear prayer may be done in several ways, according to the situation and whether the enemy is in the direction of the qiblah or not. We have explained that in the answer to question no 36896 .
And Allaah knows best.