3 . My Photo Stream is synchronising with iCloud, yes, but My Photo Stream is not necessarily identical on all devices. It will depend in the available storage on your devices, how long photos from my Photo Stream will be appearing on each device and how long they will be kept. (see the paragraph "I see different photos in the My Photo Stream album on each of my devices" in this document: Get help with My Photo Stream - Apple Support)
My Photo Stream is only a temporary buffer in iCloud. iCloud will keep at most 1000 photos in My Photo Stream and only for a month. When you first enable My Photo Steam on a device, the photos currently iCloud will download to the device. In iCloud may only be less photos than you are seeing on other devices. Other devices, where you turned on my Photo Stream longer than a month ago, may still be showing older photos in My Photo Stream, that have already been purged from iCloud.
2 . If you delete a photo from Imports and it is still visible in All Photos, you are seeing the My Photo Stream version in All Photos, not the imported version.
1 . It is difficult to compare the photos in Imports and All Photos. The Imports album is sorted by the date of the import, not by the capture date. "All Photos" is sorted also by the date of Import (on macOS 10.14 Mojave) but sorted by the capture date on macOS 10.15 catalina. Which system version do you have installed? Also, All Photos will include the photos from My Photo Stream, while Imports does not include my Photo Stream.
If you are importing photos manually from your devices, I would not use My Photo Stream at all. It can result in duplicates and missing videos and broken Live Photos. The problem is, that My Photo Stream will only transfer the still frame from a Live Photo. When you afterwards import it manually, the still frame from My Photo Stream will prevent the proper import. Photos already imported by My Photo Stream may be skipped as duplicates. Managing the imports is much easier, if you only use one method of importing, only the manual import or only iCloud Photos Library, but do not use My Photo Stream in addition to another method of import.
