That's a question you need to ask yourself. And, having seen only my own entry so far, I can't honestly tell you if that's the right thing to do or not. We're in uncharted territory here if I'm honest.
1) The planned speed is the maximum calibrated airspeed you intend the aircraft to fly at. As the aircraft climbs, it will still be able to reach that calibrated airspeed, but the true airspeed will potentially be a lot higher.
As an aside, the limit (in the US and a few other countries only) is 250 knots indicated airspeed below 10,000 feet, not true airspeed It's also something we will not concern ourselves with either in this or any other Aero Challenge round.
2) You can put anything you like for a reduction gearbox, but bear in mind that as the reduction ratio goes up, the weight and cost of the gearbox does too. For the size of the propellers in this round, I would imagine most of the entries will be fairly close to the real-world aircraft examples, with maximum propeller RPM in the 1800-2200 range.
3) Economy DOES matter - in fact, it matters more than just about anything!
4) Yes, as long as the climb gradient is in the green, it's fine. Even if it isn't in the green, it's fine - the question being would you be willing to potentially lose a lot of points in the evaluation if it isn't?
5) Since horsepower is already a major part of the evaluation of each entry, and also since horsepower is a function of torque, it's already in the challenge!