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You'd have to live somewhere without ANY inclines to get away with a singlespeed Nihola imo - even if it does come with 250w of assistance.
I had no problem picking up the inside wheel on a fully loaded postal Nihola going around corners (~100kg rider + 50-60kg of mail in the front), so tipping might a concern still.
On the contrary I've never felt like I was tipping over on our Bullitt.
Black Iron Horse might be the best option.
She does not ride any bike fast (why she wants a 3 wheeler as she feels like shell tip over on a long john) and this seems better then the dutchies like the Christiania at least to me. And this is much cheaper then the black iron horse ones she saw (8k$ here) Thinking this may be a good option but maybe even convert to single speed with a 250w ebike rear hub. I know that sounds strange but wife bikes get like that.