Um, the SS was on the flight. There was a large block of tickets bought for family together, so yes, she was flying coach together with them. There are photos. There are Tweets. For this leg of the trip, she was in a group, then for another leg she was on a private flight.
Not really even sure how this becomes a conspiracy. Is it your contention this is really a secretly pro-Trump gay couple that only Tweeted stuff to draw attention to Ivanka flying coach when she didn't really fly coach and to make her more sympathetic? Or...?
This place confuses the hell out of me.