Is it permissible to combine the Maghrib and Isha prayers for two months or more due to the lateness of the Isha prayer and its occasional absence in the summer? And is estimating the time of Isha prayer as one and a half hours after Maghrib prayer permissible and does it have a basis?

Combining two prayers while in residence was not part of the Prophet's ﷺ permanent practice. It is permissible to combine them if there is a need, provided it does not become a habit. This combination should be "pictorial" (ṣūrī), by performing the first prayer at the very end of its prescribed time and the second prayer at the very beginning of its time. There is no basis for specifying a 1.5-hour interval between Maghrib and Isha. Maghrib begins at sunset, and Isha begins when the red twilight disappears. Isha prayer should be started as soon as its time begins to encourage people to attend the congregational prayer.