You can download it from F-Droid where they will compile it from the source for you. Yes, only very few people will use that version, too. But with that logic there isn't any reason for open-source at all.

Same as every other app including Signal. For the record, not advocating using Telegram over Signal.

One would do in order to unearth any backdoors. In case of WhatsApp, that would be more difficult.