1: easy to port E2EE, it’s just math

2: browsers and E2EE is hard, you need an extension to implement it securely so the password can’t be made accessible directly to the server (you need it to remain secret even from the hosting company) or else you’re dealing with MITM risk

3: easy by sharing encryption keys using E2EE messaging protocols on top

4: encrypted search is a thing, but such indexes does tend to have some limitations

5: still easy

6: still easy, Android specifically have APIs to let apps register themselves to the file picker so they can transparently encrypt and decrypt files. But yes on other systems where 3rd party apps can’t offer such integration then it’s hard

I’ve seen one called Skiff that’s trying to do most of these things

https://skiff.com/pages https://skiff.com/drive