The following is PURE speculation and MOO:
Is it possible that Morgan had some frenemies who for whatever reason had it in for her, but not in a murderous way?
Perhaps there was some tapping/peeping activity one night. The kids in the neighborhood hear about it because Morgan's mom is well-known to be a bit over protective and over-reacting.
So they think they will have some fun with it and take up where the original "stalker" left off. Rattling doors and tapping windows, etc.
Now, suppose also that whatever the issues were with Morgan and the frenemies, they were causing M some emotional stress. She began losing weight, avoiding some of the group, etc. Perhaps she dug out those old rx meds and was taking them again to "help". (We don't know how many pills were gone, nor how many 'should' have been left when she stopped taking them two years ago)
If M was having problems with these kids and it was something she wasn't willing to share with her parents, it may have been very distressing to her. Then you have the prank "stalking", and finally her mom's over-reacting and smothering of M.
M may have had a opportunity to grab some vet meds while at the stables. She is 20, stressed out, and recognized the name of one of the drugs. Again, she is 20 and kids now are WAY more into abusing rx meds than street drugs. Even kids who don't do drugs are pretty savvy about which drugs are the "fun" ones.
So M may have just wanted to get a little "numb" to escape all the crap going on around her, and she ingested way too much by whatever method. If she knew about making her own face creams, exfoliants, etc., she may have had a mortar and pestle for that purpose and may have known how to grind pills and dissolve them into a suspension.
When Morgan is found dead, mom replays over and over the "stalking" and it just gets bigger and bigger and takes over everything until what she remembers bears little resemblance to what actually occurred.
And in light of mom's campaign, it is doubtful anyone involved in the pranks will ever come forward and confess.
Again, just a theory as to how it might have happened and it is ALL MOO.