There was a relaxed, open atmosphere where no one pried into anyone’s business, and we just had fun, light, fun conversations. There was a relaxed, open atmosphere where no one pried into anyone’s business, and we just had fun, light conversations.

There was a relaxed, open atmosphere where no one pried into anyonethe other’s business, and we just had light, fun conversations. There was a relaxed, open atmosphere where no one pried into the other’s business, and we just had light, fun conversations.

It was really comfortable, and I laughed a lot. It was really comfortable and I laughed a lot.

It was really felt comfortable, and I also laughed a lot. I felt comfortable and I also laughed a lot.

What does the “it” refer to? Is it the conversation or the atmosphere?

However, the one thing I really couldn’t stand was the lingering smell of cigarettes in the house. However, the one thing I really couldn’t stand was the lingering smell of cigarettes in the house.

However, the one thing I really couldn’t stand was the lingering smell of cigarettes in the house. However, the one thing I really couldn’t stand was the lingering smell of cigarettes in the house.

This sentence is already fine, but I'd normally put 'the' when I'm referring to the only thing I couldn't stand in a specific situation. If I was talking about myself generally, I'd say "One thing I really can't stand is [...]". Without the 'the', it implies that there could be more things.

I just can’t handle that smell — it’s the only thing that spoiled ithe evening for me. I just can’t handle that smell — it’s the only thing that spoiled the evening for me.