What is the ruling on praying Fajr in congregation in the mosque after shortening the duration of the iqama to 15 minutes after the adhan, knowing that the true dawn (Fajr al-Sadiq) begins 17 minutes after the adhan?

Prayer is not valid before its due time, whether by certainty or strong probability. If the time set by the Ministry for prayer is doubtful or precedes the actual entry of the time, then the prayer offered at that time is not valid. You must contact the Ministry to inform them of this. If they do not respond, pray at the time when you are strongly probable that it has begun. If the Imam refuses (to lead the prayer at the correct time), then you may pray in your homes.