Users can’t report it because there is no way to tell for them
Atleast the one who breached can tell? no telegram users data have been seen on dark web yet, no person/org have claimed to get any vulnerability in their system. Also if its that easy to breach why govt's keep banning telegram for not giving them userdata? despite telegram is the biggest app where most terrorist orgs operate, hub of piracy and illegal things, you can call it "public" darkweb.
Yes, pretty much necessary to provide multidevice support
True that and telegram sucks big here, but I donth think e2ee can be enabled in a feasible way for multiple devices.
The MTProto isnt open source but its fully documented, there have been security audits on it.
No. Pavel Durov have always said since starting he paid for telegram's servers from his pocket, in recent years telegram has started monetisation programs to cover its costs.
The founders were born in Russia, but they now have dual citizenship of UAE and France. If you are talking about politically questionable, even signal have been accused of having backdoors for CIA.