Many thanks for replying!
Regarding TOEFL and the Math background you mentioned, as I wrote, I have a BS in Electrical Engineering from a US university (which takes care of the TOEFL requirement) and therefore have taken as part of the core curriculum, Calc 1-3 (Calc 2 is linear algebra) plus a Differential Equations class and a Grad Level Mathematical Statistics class, my grades are really mixed (A, C, A, C, A respectively).
Concerning the holistic evaluation of undergrad and grad GPA you' re mentioning, my undergrad GPA is low throughout almost all semesters (essentially all apart from the first and last), but my grad GPA is consistently high. Now based on your analysis, assuming they give equal weight in considering both grad and undergrad GPA, then I would pass the "fizzling-out" test, since I am finishing out strong. My concern however, is that they will give far more weight in my undergrad GPA, since it is from a well known US institution, whereas my grad GPA is from a much lesser known (relatively unknown in the US to be honest) university, and hence they might trust it less as an indicator resembling potential. I was thinking that if I do a semester abroad (at a US university) and take some challenging classes earning a high GPA, that might serve as indicator reassuring them that my entire grad GPA is a genuine mirror of my potential, and not just a result of an easy grading policy by the university.
Here is the thing - I am relatively certain that unless my grad degree is given considerable consideration in the admission process, I have very little chance of getting accepted. The math background thing does represent a worry in my mind due to the inconsistent performance I've had in some of the classes, however I definitely have all the technical requirements in terms of classes, covered. Main worries are the low performance in many econ classes, and even more, the degree to which my high grad GPA will cancel out my terrible undergrad overall performance.
If you have an more thoughts on any of the above, please do add them, the advice is really helpful.