On desktop* there is no e2ee even among the options.
* where "desktop" includes GNU/Linux phones.
Desktop apps are not the ones where you create Telegram account; you only authorize them.
For another example, Threema, which is E2EE by default, doesn't allow more than one device with the account (to use multiple devices, you must create several accounts, one for each device and link them, and then basically any chat is a group chat. For desktop use, the web client uses webrtc to talk to your device to use it as a proxy).
1. I was able to use telegram without using the smartphone app at first, I just needed a phone number to register on desktop
2. The MacOS desktop app supports E2EE
3. Some third party desktop apps support E2EE
4. Signal has synced E2EE that works on both desktop and smartphone (And so does Matrix)
2. Which one? There are several and the one I'm running (the official one; brew cask telegram-desktop) does not.
3. They are third party and if it doesn't work nicely or has missing functionality ("why are not my chats from other devices there?"), Telegram devs can still point fingers that's not the official client and should be discussed with the third party developers. Ultimately, they will find out why the original Telegram doesn't do it.
4. Signal doesn't do cloud messages as Telegram does. You might have noticed that functionality like that makes Telegram much more popular among normal users.
