In the play Gertrude reports that Ophelia was standing on a willow branch overhanging the river when it broke and she fell in; here Gertrude clearly sees that Ophelia deliberately runs into the water and goes beneath the surface. It might seem like a case of Adaptation Distillation or Dies Differently in Adaptation, until one recalls that even in Hamlet we only have Gertrude's word that this is how Ophelia died. It's not hard to imagine that Gertrude tries to frame it as an accidental death rather than suicide so that Ophelia will get a proper burial and her reputation isn't further sullied, as she cares for Ophelia and blames herself for her death, hence why the details of Ophelia's onscreen death differ from Gertrude's account.
Mechtild states that she was accused of witchcraft after her baby was stillborn (or possibly miscarried late-term), with the townsfolk suspecting sorcery was involved. Seeing as Mechtild was known for dabbling in herbology and so may have known herbs and plants that could induce abortions, reading between the lines it could be insinuated that the reason she was branded a witch was due to the belief she got an abortion, or because she already realized her unborn child had died in-utero and so she was forced to induce an abortion to prevent health complications. Abortions and those who performed them were sometimes associated with witchcraft in medieval and Renaissance Europe.
We see Ophelia slipping the vial of poison she found in Claudius' cloak into her sleeve, presumably so she’ll have proof of him murdering the king. After Polonius' murder, Ophelia still has the vial on her, allowing her to drink a small amount to fake her death. Given how dire her situation is at this point and her clear distress over it until she learns Hamlet is still alive, it makes one wonder if she wasn't keeping the poison close to her for a different reason.
Taking into account that suicide was considered a grave sin back then (in Hamlet, there's controversy over Ophelia being given a proper Christian burial due to the belief she took her own life), Gertrude's death here becomes even more tragic. In Ophelia, Gertrude knowingly and willingly drinks poison to kill herself, which may mean she's so far past the Despair Event Horizon she doesn't even care if she goes to hell; she may even believe she deserves it for what she did (even though she never intended to hurt anyone).
