"There's nothing in your anecdote that disputes she could have planned it long in advance"

It seems implausible.

She lived with her sisters and while she was quite capable of many tasks, I think that long term subterfuge was beyond her. She was well into mental decline.

"You have to have 2 independent medical assessments at a minimum as well as written consent that is witnessed"

Could this not have happened at the hospital?

Well its impossible for us to know her state of mind, anything would be speculation.

Yes, it could have happened at the hospital but do you think they have people sitting around waiting to do medical assessments and be witnesses just so they can push MAID onto people? At most hospitals the doctors and medical staff are extremely busy.