You may very well be correct about the Literider's minimum. I did not look it up. Assuming that IS true, boo on Harmony!
I assume the taxis HAVE seat belts, right?
If it were me, I'd probably work on getting SOMETHING on everyone first. Educate about mom sitting in the front seat with a seat belt. Get a Scenera for the 18 mo old. And work on getting her to USE all the seats she has (including the Literider). Using the 'better when they leave than when they came' approach, if she's belting everyone in in SOMETHING, that's better than right now when she's sometimes using nothing.
If you can get her to buckle everyone up successfully, then you can work on better fitting of the harnesses. I think it's a little unrealistic given what you've written to think that she'll carry out and install 3 car seats, so maybe the Literider is better in that respect. A baseless bucket and Literider are quick and easy to get 'installed'. So that just leaves a seat for the 18 mo old to worry about.