Hi Mike,
Thank you for your comment and for creating this discussion. For all questions, the answer will be, it depends 
1- A farmer will replant if the density of plants is very low due to a bad emergency, weather damage, etc. In most cases, plant counting is done between v2 and v4.
2 and 3, will really depend on what you are trying to identify/detect. Please visit Corn & Drones: A Practical Field Guide
Lets wait for other user’s comments 