Is it permissible, in Islamic law, to adopt a system of an hour and a half between the Maghrib and Isha prayers in northern Italy, given the inability to verify the entry of the time and the sighting of stars but without certainty of the red twilight's disappearance?

You are obliged to rely on the report of a trustworthy person if you do not know when the time for 'Isha' (night prayer) begins. It is sufficient for you to act according to an approved calendar, such as the Umm Al-Qura calendar. The time between Maghrib (sunset prayer) and 'Isha' varies according to the seasons of the year. Often, an hour and a half is sufficient for the time of Maghrib to end and 'Isha' to begin, but the matter should be precisely determined according to an approved calendar.