That’s not how electron apps works. When you load a website with your web browser you get served the front and execute it. When you have an electron app, the front is in the source code of the app, and you decide when to update it so you don’t get served unexpected compromised updates. As for the paid service : They don’t sell your data and don’t show you ads so they need money, it’s that simple.

I already answered that. Yes you can’t trust a website’s content, that’s why they offer apps. It’s your choice to trust the website which is as secure as they can make it, or you simply use the apps…

I’m not sure what you’re talking about ? You’re not sending your private key to their server without first encrypting it first locally. Their servers are not doing the E2EE, your client is. The website front and apps are open source.

Yes they could send you a compromised front if you use it via their website, that’s a compromise you accept, otherwhise you could only use their apps which are open source.