Was wondering whether there was a difference of opinion on this amongst the scholars. My understanding was that a Mahram was required in this instance, however there seems to be some 'respected' islamic groups which allow women to travel without a Mahram as part of a larger party.
السلام عليكم و رحمة الله و بركاته My understanding is the same as yours, that a mahram or husband is required for a woman to travel for umrah/hajj, or any other lengthy journey for that matter. I can see your complication, but unfortunately you'll find 'respected' groups doing all sorts of things in all sorts of matters, so I wouldn't pay too much attention to the practices of others nowadays. Rather just stick to one scholar whom you find you trust, and follow their guidance, without necessarily condemning all other groups.