The purpose of photo stream is not to serve as a permanent backup for your photos, but rather to "mirror" photos taken on one device to other devices that are connected to your iCloud account that also have photo stream enabled. (Take a photo on one device, it appears on all the others. Delete a photo from photo stream on one device, it is deleted on all others.) This allows you to share your photos across devices, but if you want to keep these photos permanently, they should be saved to your camera roll so they can be imported to your computer. In fact, photos in photo stream only remain there for 30 days; after this 30-day period they are removed from photo stream although they will not be removed from the photo stream album on any devices that have already been received them. However, if any of these devices are restored or replace, and photo stream re-enabled, only the photos that fall into this 30-day window will be streamed back to the device (since the older photos are no longer in the photo stream). Unfortunately, this confusion has resulted in a number of users losing photos when they upgrade their phones and are surprised to find that only a fraction of the photo stream photos are still there when the enable photo stream on the new phone.
If you have imported the camera roll photos to your laptop, you can delete them from your phone and this will have no effect on photo stream. The photo stream album on both your phone and your iPad will continue to have the same photos as before. Since the photo stream is not effected by deleting the camera roll photos from your phone, you don't need to save the photo stream photos to the camera roll on your iPad. (That would only have been necessary if you were deleting the photo stream photos from your phone, not the camera roll photos.)
As for you iCloud account storage, you won't see the additional free space in your iCloud account until you perform your next iCloud backup. Your camera roll photos (and videos) are included in the backup so your next backup should reflect this reduction in space. To get additional details on what is taking up your iCloud account space, go to Settings>iCloud>Storage & Backup>Manage Storage. Here you will see the space being taken up by your Backup, Mail, app Documents and Data. If you tap your backup you will also see how much space is being taken up by various items contained in your most recent backup, and have the ability to selectively exclude items from the backup to reduce its size. This article outlines methods for managing your iCloud space: http://support.apple.com/kb/HT4847.