は vs が in neutral sentences

Question

I have a couple of situations I was hoping someone could help me understand if が or は is more appropriate in the following situations.

Scenario A: Describing what a person is doing.

1. You are by yourself and you see a complete stranger buying food and for whatever reason you feel like muttering "That person is buying food over there".
2. You are with a friend who also sees the stranger and also doesn't know who the stranger is and you want to let your friend know and say to them "That person is buying food over there."
3. You see celebrity Ariana Grande who you know about because you listen to their music (but don't know them personally) and they are buying food and you want to mutter to yourself "Ariana Grande is buying food over there".
4. You see celebrity Ariana Grande buying food. You're with your friend who you believe is also familiar with Ariana Grande and you want to let your friend know and say to them "Ariana Grande is buying food over there".
5. You see celebrity Ariana Grande buying food. You're with your friend and you believe they wouldn't know who Ariana Grande is because they don't listen to pop music and you want to let your friend know and say to them "Ariana Grande is buying food over there."
6. You are with your friend and you see another person who is actually John and is friends to you both so you both know who John is. John is buying food. You want to let your friend know and say to them "John is buying food over there."

Scenario B: Describing a person's features

1. You are by yourself and see a complete stranger with red hair and for whatever reason you feel like muttering "That person has red hair."
2. You are with a friend and you see a complete stranger with red hair and you want to let your friend know who also doesn't know the stranger and you want to say to them "That person has red hair."
3. You see celebrity Ariana Grande with red hair who you know of and want to mutter "Ariana Grande has red hair."
4. You see celebrity Ariana Grande with red hair. You're with your friend who you believe is also familiar with Ariana Grande and you want to let them know and say to them "Ariana Grande has red hair."
5. You see celebrity Ariana Grande with red hair. You're with your friend who doesn't know who Ariana Grande is and you want to let them know and say to them "Ariana Grande has red hair."
6. You are with your friend and you see another person who is actually a friend called John and he is actually friends to you both so you both know who John is. John has red hair. You want to let your friend know and say to them "John has red hair."

Answer 1

A1. You are by yourself and you see a complete stranger buying food and for whatever reason you feel like muttering "That person is buying food over there".

zero particle; あの人、あそこで食べ物買ってる (if you don't specify him with あの, it's が; あそこで人が食べ物を買ってる)

A2. You are with a friend who also sees the stranger and also doesn't know who the stranger is and you want to let your friend know and say to them "That person is buying food over there."

If you use あの人 in the sense of him or her rather than "that person", you can use が besides zero particle; あの人（が）あそこで食べ物買ってる.

As for A3-A6, it's が.

B1 and B2 are either は or zero particle. i.e. あの人（は）髪（が）赤いな.

When it comes to B3-B6, it's は, unless they have abruptly dyed their hair.

Edit:

Scenario A depicts the scene where you discover a temporal or dynamic state which is unusual and unexpected. When you describe or report that, you don't use any topic phrases to compose a sentence. As a result, you use が to denote the subject. (You can search this site with "現象文 or neutral descriptive が".)

However, when the subject of that type of sentence is modified with a determiner like この, その or あの, the subject has to be marked with zero particle. In other words, you don't add any particle after the subject. (Incidentally, question of 現象文 is accompanied with zero particle too.) In this regard, あの人 can be a simple personal pronoun which corresponds with him or her. When you use it in that sense, it's a bit different from adding a determiner to the subject.

Scenario B refers to something to some extent permanent. So, you use topic phrases, which is a principle.