It is not possible to add images to an existing project but you can certainly upload images from multiple flights to be processed all together as one.
If you have to have your data turn out well be sure you are following all of the Data Collection guidelines very carefully: https://www.mapsmadeeasy.com/data_collection
1. They will work as long as the lighting conditions are similar. If you collect a section during bright sun and an adjacent section during heavy clouds there may be an issue.
2. All outputs get created all the time. There is no discount available for not creating the advanced outputs.
3. Our pricing is by the megapixel, so yes, it involves the number of images. NumberOfImage X CameraResolutionMegaPixels / 5 = Points