Wow, I read your post and being an engineer, I am kinda overwhelmed with the details. I am comfortable with modding electronics and mechanical. just new to bikes and its lingo.

Excuse me for my basic knowledge. What is freewheel? Are you saying that they typical stock wheel or standard Dahon wheel is not able to change. Because I see 3 speed cartridges as standalone but do not know how to verify which wheels it can fit.

Reading your posts, seems like you are doing non-standard Dahon mods. This is more than I want to do as I do not want to rebuild a wheel and experiment with parts as this will get a little costly and time consuming and lack of space, I live in a city apt.