I'll answer the questions I feel I am qualified to answer :)
How much should I request from the DHF for the prize pool?
@fredrikaa's suggestion sounds great.
Should I create a proposal for marketing the hackathon?
Yes. A separate proposal to keep it transparent.
How many winners should be eligible for the prize pool?
With two phases, 10 to qualify for the second round, 3 winners.
With one phase, 5 winners.
One phase or two phases?
Got any ideas to make the Hackathon better?
Not for the moment.
Would you be participating?
Yes, as a one-man-band without any hope to qualify for the second phase :)
RE: STEMGeeks Second Hackathon