If I delete my iCloud photo library from my iPhone in order to free up storage and I have my photos backed up to the Photos app on my Mac, will the photos be deleted from the Photos app?

The iCloud Photo Library is disabled on my iPhone currently.

Then you can delete photos from the iPhone without deleting them from the other devices still syncing with iCloud. But not delete photos directly from iCloud to free iCloud Storage.

If I transferred the photos (I have about 2,400) to the iPhoto application, would that prevent them from being deleted?

Yes, or make a copy of your Photos Library. This will work, if your Ma is not using "Optimize Storage" and all original photos have been downloaded to the library on the Mac and are not only in iCloud. Only one Photos Library can sync with iCloud - the one that is the System Photo Library.

Is the Photos Library on your MAc also syncing with iCloud Photo Library? then yes, if you delete from iCloud Photo Library on any device, the photos will be deleted from your Mac too.

If you want to delete photos from any device, without deleting them from other devices, disable iCloud Photo Library on that device, before you delete photos.But if you want to delete photos from iCloud without deleting them from other devices, turn off iCloud Photo Library on the other devices, so the delion on iCloud cannot sync to the other devices.