If there is no record of her getting on a flight, I think she may have gone to Newark to get transportation to Penn Station or an Amtrak train to go somewhere. She had already been in NYC for 4 days, it seems, from May 14 when she arrived at LaGuardia to May 18 when she was seen being dropped off by someone at Newark airport. I'm going to go with Occam's razor on this case, with the simplest theory being the most probable. I doubt she has a fake passport and has used that to travel. I think she got lucky because she did all this before the arrrest warrant was issued on May 19th, if I've got the timeline right. I think she probably took the train somewhere, where is hard to say, of course. I think she would have gone somewhere that was familiar to her. Maybe she did take a train to Detroit where she grew up and knows the area, and maybe growing up in Detroit area she crossed over into Windsor, Ontario frequently, and if this was before the arrest warrant was issued, maybe she took a train to WIndsor. Just thinking out loud. If she did cross into Canada by train, perhaps there is less likely to be a record. If she didn't go by train into Canada, then perhaps she took a train to upstate New York or somewhere else on the East Coast that she is familiar with.
It may be that LE just have a camera image of a car dropping her off, not the license number, etc. and I think she might have taken an Uber from wherever she was staying near NYC after she landed in LaGuardia. She must have had a purpose in going to NYC. I doubt it was a random destination. Unless she just wanted to be lost in the crowd until she figured out her next steps, and she thought that NYC would be a safe place to do this.
IIRC, CS said the last time he spoke to KA was May 13th. But she flew from Austin to Houston and then to LaGuardia on May 14th. Did she spend the night at their home in Austin on May 13th? Two days after the murder of MW, and after her LE interview?
I am trying to focus on the timeline and what she might have had to do each day/night.
I don't think the killing of MW was pre-planned to the extent of passports, going on the run, taking money out of the bank, etc. It may have been premeditated from an emotional perspective, but I think she had to think fast afterward, and is just lucky that the US Marshall's warrant took so long to be issued.