So, I never got a good introduction to 万万, I always just saw it pop up on test questions next to 千万 as one of the answer choices. For a long time I never got a good explanation of it either. I also never ever heard any one say it in real life or even in teaching material "dialogues", etc.) and almost never saw it written. One day I decided to look it up and it turns out, 万万 really doesn't have anything to do with 千万. To a native English speaker 千万 feels like something, it has a strong feeling of suggestion and exhortation for a positive or negative thing. 万万 on the other hand is used only in the negative sense with a meaning and usage equal of 完全
我完全不同意。 = 我万万不同意。
My dictionary has the line 那是万万不行的。 When I look at this line all I can think is that any person would more likely say 那是完全不行的, or maybe its just me.
And yes, 万万等于一亿， in some older books you will actually not see the character 亿， this character may have been a later invention, who knows.