Learn English – Difference: who cleans, who cleaned, and who has cleaned

In situation 1, which version you select (they're both grammatically correct) depends on the tense in which you're telling the story. If you're telling the story in the present tense ("So I'm walking along the street..."), you would use option A to match, while if you're telling it in the past tense ("So I was walking along the street..."), you'd use option B.

In situation 2, you'll generally want either A or C.

The essence of the grammar article was probably that in English, it's possible to discuss either past events or hypothetical future events in the present tense, as if you were placing yourself in the time and narrating from that perspective. In this case, you'd use the present-tense "first time I've been", but it's much more common to narrate this way when relating past events than when talking about future plans.

Have as you used it, it's the present perfect, where If you use be+-ed you usually form the Passive form.

As you can imagine

is come

doesn't make sense, because that's an active action he does. Instead

Lunch is ready

is ok, because it's something "Lunch" suffers

If you want more detailed information you can check this resource.

I would say:

1. Tea has come
2. Lunch is ready
3. He has come back
4. She is assigned for work