Ok, let me break it down because clearly I didn’t explain it well.

What is supposed to happen, scenario 1: the client encrypts your messages with the public key of the recipient, sends it to the servers of WhatsApp (or whatever service) along with some encrypted metadata indicating the recipient, which then forward the message to the recipient.

What could happen, scenario 2: the client does the same, but also encrypts another copy of your message with a public key that belongs to WhatsApp, and send both versions to the WhatsApp servers. They decrypt and keep the second version while forwarding the first one to the recipient.

Or, scenario 3: they just never bother with end-to-end encryption, and always encrypt it with the WhatsApp key, still sending it to their servers which then reencrypt with the recipient’s key before forwarding.

In all cases, messages are sent only to the WhatsApp servers, not two places. The only visible difference is in scenario 2 where the communication is larger. You can’t inspect the metadata of the message with your network sniffer, because it is also encrypted, so there’s no way to rule out scenario 3.

If the protocol is designed to be transparent by not encrypting the entire payload sent to the servers, and you have access to the recipient’s private key (those are big ifs) then you could show that there is indeed an end-to-end encrypted message in there. But this is true for how many of these proprietary services? Maybe for WhatsApp.

Just encrypt it before sending it to their servers. How would you tell that apart from any other traffic it sends? (E.g. to check for new messages, to update who of your contacts is online, etc)

Almost all services in that list are closed source, so even if they use end-to-end encryption nothing stops the client from sending all your messages to anyone they like after decrypting (in fact some of them already have it as a built-in feature in the form of backups).

How would you even hit a 3.3 TB limit a month in normal usage? AAA games these days are hitting 100 GB but how many of those are you going to download in a month? Streaming Netflix 24/7 will also not get you there, unless maybe it was 4k content the whole time. Maybe if you’re pirating uncompressed Blu-ray rips?