Any suggestions for mother with Alzheimer’s & Parkinsonism with leg weakness, tremors, and inability to walk or even stand?

It sounds to me that you are fast approaching the point that even a sit to stand is beyond her abilities since that requires she bear weight, the only other option is a full lift with a toileting sling, either portable or ceiling mounted.
BTW, how are you managing to bathe her?

Discuss this leg stiffness with Doctor. Maybe med review or even re-timing may help?

Do you have a Physiotherapist who visits? It may be time to review the safest way to transfer? Have a plan for good days & for stiff days.

I'd be very reluctant to move her when legs very stiff as I'd be worried about causing injury - this is where Physio could advise. If can't get up, use bed pans - roll on/off in bed instead. Do you have a hospital bed that lifts to your hip height? If not, you will need one to save your back.

I am thinking if your Mother cannot weight bear on her legs safely (either that day or more permanently) she will need to be transfered using a hoist instead - from bed to commode or wheelchair. Although stiffness in the hips may prevent that too - as quite a full bend is required.

Are you on your own? 2 x assist would be much easier.

I don't have experience with your situation but just want to point out (since you mentioned it in your posted) that a supplement is still a chemical that can react with any other meds your mother is given and therefore must be discussed with her doctor or pharmacist before giving it to her. I hope you get some good solutions to your question.