Please check these.

[Anonymous]

Hi, Rusty and Pete!
Long time no see. How have you been? I was abroad for a month, which was why I couldn't come here. I've missed you.

Would you please check and correct these sentences?
1. [At a bookstore] Get a book, so/then you can have the writer's signature. [Which is more idiomatic, so or then?]
2. She was masquerading as a nymph.
3. He is bumming around the/his house, six months [off the job, between jobs].
4. I think this's not a thing/problem/matter to laugh it up. His joke was way [too much, out of line, over the line].
5. Nowadays we live in an rapidly-changing world, in which we should always be alert in new information and technologies.
6. That was a different Jack from the one I had loved so passionately before.

Happy new year in the lunar calendar system.
Ann

[Pete]

1. [At a bookstore] Get a book, so/then you can have the writer's signature. [Which is more idiomatic, so or then?]
-- The meanings are different; the punctuation is different as well:
- Get a book, so you can have the writer's signature. [You are giving a reason for the person to buy a book.]
- Get a book; then you can have the writer's signature. [You are giving a consequence of buying the book. It's the same as, "If you get a book, you can have ..."]

2. She was masquerading as a nymph. [OK.]
3. He is bumming around the/his house, six months [off the job, between jobs]. [All OK. Probably "the house", is more common, and it is presumed to mean the house where he lives.]
4. I think this's not a thing/problem/matter to laugh it up. His joke was way [<strike>too much</strike>, out of line, over the line].
5. Nowadays we live in <strike>an</strike> a rapidly changing world, in which we should always be alert <strike>in</strike> for new information and technologies. [I don't think you need the hyphen in [i]rapidly changing.
6. That was a different Jack from the one I had loved so passionately before. [OK. Also common and correct is: "different ... than".

-- p.s. Nice to see you back, Ann. I hope your trip was enjoyable.

[Anonymous]

Thank you very much.
All the best.
Ann