Is it possible to change API coefficients for several selections at once?
There is no such functionality, mass replacement is possible only via CSV.
Is it possible to delete campaigns via API?
You can delete campaigns via API. You can see how to do this and what other functionality is available at the link.
Where can I get a list of devices for the "Device Manufacturer" item when creating a campaign via API in the "Microbidding" section?
To solve this issue, you need to use the following request in the API: "Get a directory for creating a campaign". Then you need to click "Device manufacturer". After executing the request, you will receive a response with all the necessary data.