Cool, I've found a future alternative, from no one else but the CryptPad team: spreadsheets are among their [goals][1], but will require quite a lot of time and funding, I'm continuing to look for a current alternative, but this is great news
[1]: https://blog.cryptpad.fr/2018/07/13/CryptPad-Roadmap/#Goal-1...
Not sure you need it (and not ready yet), but I am including e2e encryption into my 1:1 meeting note-taking app: https://www.oneonemeeting.com
I am working on the feature right now, but as it is still a side-project, it will take some time.
Why are you trying to encrypt online spreadsheets? It's very unlikely that you're going to get much operational security out of the encryption features in a spreadsheet application; you will get far more security out of careful use of sharing and authorization controls, which are things you can get from Google Docs and not so much from other platforms.
Remember too, if the tool you're looking at works entirely in a browser, without an extension (sometimes even with an extension!), it's not "end-to-end encrypted"; it's as encrypted as the server wants you to be as you run it.