Is the planned annihilation of humanity by the Extraterrestrial Alliance to be a very precise, very surgical culling of humans while preserving as much animal and plant life as possible?

It is surgical only to the extent humans are the target. So the actions taken will be focused on them entirely. There will be collateral damage as well on wildlife. This is of no concern to the extraterrestrials. There are many sources of creatures in the universe that can be used to populate a planet if that is desired. Keep in mind these beings have no real appreciation for the wonders of life and do not value animals any more than human, except as a potential source of food. They do not take appreciation in the beauties of nature because they are loveless beings and that fine appreciation is an aspect of love and love energy.