Photostream will store the previous 30 days worth of photos automatically. That means, every photo you take on your iOS devices will be sent to Photostream, but only the last 30 days worth of photos will be available there.
If you enable 'Automatic Import' of Photostream in iPhoto (Preferences > Photostream > Automatic Import) then when you launch iPhoto, it will automatically download a copy of any photos that are in your Photostream, and automatically create an Event titled: {MMM YYYY} Photo Stream (eg. "Jul 2012 Photo Stream")
This Event will be automatically kept up to date, as long as iPhoto is open long enough to sync everything required (depends on your connection).
In effect, as long as you open iPhoto once every 30 days and leave it open long enough for your Photostream to be imported, you should never have to manually import a photo from your iOS device at all.
As for removing items from your Camera Roll on your iOS device, as long as you see that they are present in Photostream (or in iPhoto), then it is safe to delete them from your device. (There is no way around this but to do it manually as far as I know.)
Currently, Photostream does not support videos. To import videos, connect your iOS device to iPhoto and import these manually.
As a side note, in order to keep some photos on the device, you can create a smart album in iPhoto along the lines of "Photos taken within the last 30 days" or "Photos rated 4 stars or more", and configure iTunes to sync these to your device. That way, you can be sure that no matter if you clear your Camera Roll on your device, or delete certain photos, you maintain some convenience.
First question: yes, iCloud backs up your photo as it is. If you delete media from your photos library it will be deleted from iCloud. (I might add that videos are also stored in the cloud - and consequently take up the most room).
Second question: It is possible you can use the Scanner and Camera wizard to import photos from your iPhone. Here is are two articles that briefly describe the process.
Before you delete all (or most of) your photos off your iPhone, I would recommend that you double check that everything has been transfered correctly so you don't lose any pictures.
You probably already know this but it's worth mentioning anyways. When you go to delete photos from your iPhone, open the Photos app, click the Camera Roll album, then select the box with the arrow icon at the top right of the screen. Select pictures to be deleted, then tap the Delete button in the bottom bar (this is also a way to email more then one picture).
Of course, the other option would be to turn off the iCloud backup of your photos. (You already know how to get there but go to iCloud > Storage & Backup > Manage Storage > [your name] iPhone > Turn Camera Off.)
Best Answer
What i think the best solution is: Go to Whatsapp -> Settings -> Chats. Enable Save to camera roll. From now, all your incoming photos and videos will be saved automatically into your Photo app. The name of the album with the saved photos and videos is called WhatsApp. But the incoming photos and videos are also saved locally in the WhatsaApp app.
So now you should enable iCloud Photos in Settings -> Your Name -> iCloud -> Photos. All your photo albums will be backupped in the iCloud. It is not possible to backup one specific album.
After the iCloud backup, the photos and videos are still saved locally on your phone in your WhatsApp app. To delete this, go to WhatsApp -> Settings -> Data and storage usage -> Storage usage and choose a chat. From here you are able to delete the media which is stored locally in the WhatsApp app.