I see that you have some questions about deleting photos and storing them in iCloud. I can help shed some light on this.
iCloud is a syncing service, so photos that are deleted from the device with iCloud Photo Library will also delete from iCloud. You can turn on optimize photo storage to keep lower resolution photos on your device, and larger resolution photos in iCloud. This can help save space on your iPhone. Here's some info on this:
Get started with iCloud Photo Library:
- Tap Settings > [your name] > iCloud > Photos. If you're using iOS 10.2 or earlier, tap Settings > iCloud > Photos.
- Turn on iCloud Photo Library.
- Select Optimize iPhone Storage to make more space on your device.
With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. And as long as you have enough space in iCloud, you can store as many photos and videos as you want.
Manage your photo and video storage
One alternative is you could import your photos to a computer for safekeeping, then delete them from the device/iCloud: Transfer photos and videos from your iPhone, iPad, or iPod touch
Take care.