Hi Richard, Welcome to JustAnswer. I'm a licensed veterinarian and I'll be happy to help. First of all I am so sorry for your loss of Molly. I'm sure in a way Mitzi has also felt the stress of her loss. You are doing all of the right things for Mitzi. . .playing with her and giving her lots of love. . .but the stress of Molly's loss may have started a FLUTD in Mitzi. These letters stand for Feline Lower Urinary Tract Disease, and one of the main causes of this seems to be stress.
Also, being in the states, I'm not familiar with the term "pedal bin"? If you mean a garbage can, then if you can, I would recommend removing it and cleaning the area thoroughly with an enzymatic cleanser so that you are also eliminating any odor that Mitzi can detect.
I also recommend that you have her checked for FLUTD to make sure she doesn't have a medical problem. I really appreciate the fact that you have altered the litter trays for her, but if she was used to a covered litter tray with a certain kind of litter, then I would stick to that. The fact that she does defecate normally in the litter trays that are available, means that her problem is not litter aversion.
I'd have her checked out medically, and if all is well, then your vet may recommend using an oral anti-depressant temporarily to help Mitzi through her grieving.
Ultimately, to "retrain" Mitzi to using the litter for urination, you may need to confine her to a bathroom (or other small room) without and "pedal bins" but with her litter (s). . .you could put both a covered and uncovered litter to see what she would prefer.
I hope you find these suggestions helpful. If you have any further questions or concerns just let me know.