Natural causes is not a likely cause for a healthy 34 yr old. It's possible of course, but not probable, and it doesn't explain the preamble of weird behavior that her family observed. Sometimes people with an infection behave in strange ways, sometimes people with cancer behave like this (perineoplastic syndrome). This is not the average though, these are the exceptions.
While the toxicology may not be able to accurately determine the plasma concentration of whatever it was there is a good chance that the presence of a substance can be established and also perhaps the boundaries of its range. The limit of detection for many drugs of abuse is single parts per billion. Their presence can be inferred from their decomposition products as well.
There may also be other evidence such as residue on the lips or in her nose, an injection site, a syringe, a bottle or bag with left over material or residue, testimony from whomever she met with... Who knows what else they found in the car or with her luggage.
The ME will put all of that together.
Its entirely possible this was an unintentional interaction.
I'm am not saying that you're wrong, of course, we are all just guessing on very limited information.