P: Online Catalogs

With current Lightroom catalog structure having it only in the cloud will be too slow for working comfortably. They create tons of little files inside, and that's why I always recommend to put the catalog on an SSD, while RAW files can stay on a slow HDD without any problems. If they change something it would be great to have catalogs in the cloud. Anyway the catalog itself is pretty small - only the previews and smart previews consume space. You can always delete them and build them again when needed. Most of the space is consumed by RAW files, but not the catalog. You can store RAW on any cloud storage, including Amazon Prime where (as far as I know) are unlimited options for photo storage, including RAW and DNG.

Specifically for your usecase (remote files that are not stored locally, but can be edited), none of the usual suggestions will work. Google, OneDrive, Creative Cloud Documents folder, all sync with what is on your local drive.
There is only one service I know of that works how you want it to... sort of. Box drive. https://www.box.com/drive

It has a folder location that uploads to the cloud, then erases from your drive but leaves a kind of ghost file behind. It re-downloads the file on-demand. Works great for small stuff, but if you have 500mb or 2gb PSD files it can take a long time to "open" because you are downloading it from the cloud first.

I use dropbox and love it.  I have 3tb for about $199/year and have all my documents, photos and everything inside my main dropbox folder organized to my liking.  When I make any changes to any document, including saving from LR to dropbox they automatically get uploaded to the web when I am connected.  I have 2 tb on my computer and need to rethink how I will deal with it when this fills.  Dropbox does have an option to keep a cloud copy only to relieve the space on your hard drive.  I need to think about this.  I don't keep a lot in Lightroom because I'm already paying for dropbox.  It's easy to share folders, etc.  I don't know much about the others you mentioned.

To the extent that these cloud storage services map and sync to a local folder on a local drive, you can do this. However, the Previews folder which can get huge, also needs to reside in the same folder as the catalog file. While you can do some geeky tricks to fool the OS about the location of the Previews folders, it is not recommended.
You also have to make sure that any LR temporary files don’t get synced to the cloud service And that your catalog is closed and updated before you use other devices that share the cloud service sync