Is it permissible for a mature woman to go for Umrah without a Mahram, if she is accompanied by trustworthy women and it is an obligatory Umrah by plane?

The fundamental principle is that a woman is not permitted to travel except with a mahram or her husband. An exception to this is travel for an obligatory act of worship, such as the obligatory Hajj or Umrah. In such cases, she may travel with a trusted group if she cannot find a mahram or her husband. This is the view of the Maliki school, one opinion among the Shafi'is, and the choice of Shaykh al-Islam Ibn Taymiyyah. The evidence for this is what is stated in Mawahib al-Jalil by Al-Hattab al-Maliki, quoting from Al-Muwatta' by Imam Malik. Shaykh al-Islam Ibn Taymiyyah also holds that a woman may travel for any act of obedience.