Assuming that the study estimating the 15 animals killed per ha of harvested crop is this one, it must be noted that the study was widely criticized for these and other mistakes and I would not in any case take it at face value.
To address the numbers mentioned in the question:
Davis estimates that 15 wild animals per hectare per year are killed
as a result of harvesting annual crops, and guesses that maybe half
that, or 7.5 animals per hectare per year, are killed on grazed land
with managed perennial forage.
Now, in the same paragraph, the methodology for this calculation is mentioned:
He does this by averaging a mortality rate from the English mouse
study (including animals killed by predators in the week following
harvest), and a mortality rate from a study of a number of rats killed
in sugarcane harvesting.
The article further admits possibility of inaccuracy of these estimates but argues that untill we have actual data they should be fine.
They actually do not seem to be, though. Apparently, out of 33 English mice living in fields fitted with radio collar, only 3 of these were killed by a combain. More than a half of them were killed by predators - these were argued to be more likely to catch them because of the lack of cover. A lot of the other deaths were attributed to stubble burning, a practice that is no longer in widespread use and restricted by governemnts.
Even if the above-mentioned numbers were absolutely correct though, there are still arguments that make veganism an absolute no-brainer when it comes to adhering to LHP (Least Harm Principle).
There is a huge difference (I believe) in accidental death or suffering and the focused, cold-hearted and intentional enslavement, violence and mass killing of agricultural animals.
The former is a by-product of actions that can be further reduced by conscious efforts and improved practices (see restrictions on stub burning above). It does not target specific species and does not make it impossible for these animals (as a group) to escape or adapt.
The latter is a targeted effort of our species which sees billions of animals imprisoned, inseminated, held in terrible conditions, being treated as things with no other real value besides the dollar that they are later exchanged for. It means that these animals are forcibly bred (often selectively, making animals unable to support their own weights because of the maximization of muscle as a result of that) and forced to watch their offspring going through the exact same thing. Well, they usually never really see their offspring because it is taken from them as soon as possible so yeah, there is that. There is no escape for these animals and even if there was, they have been so deformed by humanity's farming practices that their chances for survival without humans are.. slim. By definition, there is nothing that can be done to reduce most of the things I mention above. There can be improvements in taking care of these animals but they are often not economically viable and the invisible hand of the market stubs them out in their beginnings.
I think it is upon every one of us to decide what the least harm is in this situation. It may depend on your definition of harm or on your willingness to ackowledge things (on both sides of debate, please call me out on anything I might have omitted). For me though, the answer is quite clear.