2. You can set the speed to whatever you want but the camera is physically limited to only be able to take 1 image every 2 seconds. Depending on your flight height, overlap, and camera focal length the camera is what is setting your max flight speed. We don't generally recommend using GSPro because it lets you do so many things wrong. As with anything, do some small trials first so you really understand what is going on before doing a huge data collection effort. You should break that area up into 50 hect sections. The forward overlap value is what is slowing things down. We would recommend using 75/75 for this type of mission but you could use 70/80 which would speed up the flight a bit.
3. Overlap is one thing. Generally the more the better. But for weed identification GSD is likely going to be more of the deciding factor. The more pixels you have on target the easier it will be to make determinations. This means flying lower or using a narrower field of view camera. The real answer is testing testing testing.