It depends on what you mean by "as a whole". I mean that Telegram by default can read all your private chats, unless you manually enabled e2ee and suffer from related bad UX. On Linux desktop (and phones) it doesn't even allow to enable e2ee at all.
Yes, this is the expectation. You use someone else's platform that doesn't have E2E, you assume they can read your messages and will help law enforcement to do the same. No surprise there.
Doesn't mean their E2E feature isn't secure, or that the platform as a whole isn't secure. Facebook surely shares their Facebook + Whatsapp data with US law enforcement, we wouldn't call Facebook/Whatsapp insecure just because of that.
I and many others certainly would.
There's "New secret chat" option on my Android client. Or what do you mean by "phones" here?
I mean, if nothing else, that's a bad default. This makes it worse than, say, WhatsApp or the apple messaging thing, nevermind the likes of Matrix or Signal.
I think you might confuse what the mud puddle test aims to demonstrate. It's to be able to confirm E2E encryption, which if you do that test with Telegram + Secret Chats (which is the E2E encryption feature in Telegram), you'll see you cannot recover those messages.