In the AJ Hadsell case, research was presented into how drugs can be detected long after death via hair analysis. They can tell how much, and for how long and still provide an educated guess as to whether there was a toxic or lethal amount in the deceased at the time of death, but
not when the last dose was taken. For more information, see
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2639787/
https://www.sciencedirect.com/science/article/pii/S2090536X15000994
As a former addict here stated, long-term and ongoing users often have what would be considered lethal amounts of heroin in their systems due to tolerance. I think we need to be careful when making statements that she used right before she died, or she would have been feeling X. None of us know what Natalie was thinking, feeling or using before she was killed other than what any written evidence LE can dig up tells us. So far, that evidence hasn't produced a Craigslist ad to corroborate the killer's story, and hasn't produced any texts or notes about suicide. I take that to mean none of the texts exchanged between Natalie and the perp made any reference to dying, the hit, the ad or suicide.
IMO craigslist will turn out to have nothing to do with this and it will involve a forum or website where the perps fantasy persona met Natalie (or her persona). If there is one thing we have learned about Natalie, it is that she unwittingly befriended creeps online that violated her boundaries.