All of your sentences are grammatically correct, bravo! (Except two typos)
If anything, some strike me as a little odd logically and culturally…
2 strikes me as contradictory:
--> It's such lovely weather.
Not really… it's actually pretty cold.
3. Excuse me, is this seat taken?
--> No, I don't think so, or simply, "no". (I think it is odd to give someone permission to do something when the question was whether the seat is taken or not)
4. --> (I am your classmate) from high school (no need "time")
Why do you look so old now? --> This is grammatically correct but no one in polite society in America would say that to another person they have not seen in a long time! :)
5 --> the sentences don't seem to be related…
Can I get to know you? (I capitalized)
Why hasn't she shown up yet? (shown with an n)
6 This is very odd, culturally speaking. I have never heard a child ask specifically for "mother's milk."
The mother's response is also somewhat odd.
The language is too "adult-like." It sounds as if a mother is preparing to breastfeed a son who is no longer a child…
A child who still breastfeeds is usually still unable to speak in complete sentences, let alone to ask for "mother's milk."
The sentences are all quite well-formed, but some just seem a little odd logically or culturally when said in English. :)