Hi BigBird,
Welcome to the forums.
Tell needs the indirect object too - You usually need to tell him or tell me or tell someone. The exception would be "I'm going to tell a story." #3 is a little like that - tell the story of what you encountered.
1. If you don't know it, just say "I don't know" or "I am not sure." You're not conveying information here, just saying certain words.
2. If I'm wrong, just tell me I'm wrong.
3. I'll just tell what I encountered. This is okay, but you can also say "I'll just tell you what..."
4. Well, just to tell you that I'm a bit tired of doing this. I'm not sure what you mean by this one - it's not a sentence. Is it "I just want to say I'm tired of this"?
5. Saying I'm sorry, hurting, just wanna tell you I still love you.
6. Hello, just to tell I got a mail about excessive bandwidth usage. See #4 above.
7. John tells "I love Mary." This is really unnatural. Your confustion about whether it means that John tells everyone of his love for Mary is expected from such a weird sentence. Did you read it somewhere? "John told us all, "I love Mary."