Can someone help me with some grammar questions?

I’m revising a story that I wrote (for fun, not for class), and I have a few questions:

1. Commas for two adjectives describing the same thing?
e.g. She had long blonde hair. vs. She had long, blonde hair.

2. Hyphens in terms that include two words?
e.g. There was a bubble gum craze. vs. There was a bubble-gum craze.
e.g. I am a sixth grade student. vs. I am a sixth-grade student.

3. Run-on sentences.
There are sentences that might appear run-on but would make sense as one sentence instead of two.
e.g. He did not like to dance, but everyone else was dancing so he joined in.
e.g. I felt I should do something, but the two guys looked vicious and I did not want to get hurt.

4. Comma or semicolon?
e.g. I loved him, but he hated me. vs. I loved him; but he hated me. (A teacher told me that a semicolon was right, but I think the comma was right).
e.g. Although he gave me a book, I did not read it, and I just put it on my shelf. vs. Although he gave me a book, I did not read it; and I just put it on my shelf.

5. What is the difference between will and would? (the rest of the story is in past tense).
e.g. He promised that he will give it to me. vs. He promised that he would give it to me.
e.g. They will go to the yard after school. vs. They would go to the yard after school. (They do not usually go to the yard.)

6. How do you indicate thought? Quotes? Italics?

7. (this is a real stumper for me) How to best write this: “Once in a while, she will sit with whoever does not mind having her sit next to them.” There is at least one grammatical error here, it seems. Maybe two.

Thanks in advance for your time and your help!