1. the mathematics required for writing thesis should be independent of what language the thesis is in, theortically. this depends more on your content.
and from your description, it is clear that you dont have enough mathematics preparation at all.
My suggestion in case you are going for economics: forget about PhD at this moment. Go for a MS first.
2. this depends if they require a writing sample in the application package or not. Various from school to school. Most likely no required or optional.
3. GRE is very common reuqirement. Take it. If you are not able to do so, I think you need another plan other than PhD.
4. Also okay indeed. just check if your IELTS score pass the reuqiremnt of each school will be fine.
5. Yes. Dont dream for UK schloarship for MPhi/PhD. Not something impossible, but difficult.
6. no one care your publishcation in chinese, even you get 100 papers there. can skip this.
indeed, if you have too much low-tier paper, it will be likely viewed as 學術灌水, which is harmful to your application indeed.
7. focus on what you are interested in. this is why you wanna do a reseach master /PhD.
8. Background not good enough + want famous school + dont want master again = dreaming.
please go to the Mars, or go back home sleep if you insist all three.
for MPhil, the funding again......depends on school. but I will say it is somehow between MSc (rarely funded) and PhD (fully funded).
9. well. take it easy. this doesn't depends on zour education qualification only, but more likely your personality.