A) The use of herbicides anywhere near ponds, rivers, streams or watercourses requires great care to avoid long-term damage to the environment. Fortunately, there are herbicides that are suitable for the kind of application you require. I would recommend a glyphosate product such as Roundup or the more specialist herbicide AquaMaster applied with a knapsack sprayer.
Glyphosate is a broad-spectrum herbicide ingredient that will kill green and actively growing vegetation, so is best used in spring and summer. However, it is very safe in soil and water, where it binds tightly to silt particles before being rapidly biodegraded. It is also safe for use in the presence of fish, aquatic insects and wildfowl.
Though control of waterside vegetation will be helpful in maintaining open water areas in your flightpond, it will not replace regular desilting, which is necessary to sustain the depth of water you require. When cropping and ground conditions allow, get a digger in to clean out any heavily silted parts of the water.