What is the ruling on a woman traveling for Umrah without a Mahram, and does the aspect of reward or sin outweigh the other, knowing that she is currently unable to perform Hajj?

The preponderant view is the permissibility of a woman traveling for Umrah with trustworthy companions. This is the madhhab of Malik and Shafi'i, and it was favored by Ibn Taymiyyah. Therefore, if a mahram is not available, there is no harm in her traveling for Umrah accompanied by trustworthy companions, while adhering to the Islamic guidelines of modesty and avoiding contact with non-mahram men.