While putting this together, I came to terms with the fact that Singlish is probably far more complex than any of us consciously might be able to describe. There are all sorts of additional nuances and additional use cases for all of these terms. There are exceptions to practically all of the following rules.

It's a humbling reminder language is a living, organic thing, and it does whatever the heck it wants. How like that? Like that lor.

Lah!

A: “Hey, could you share your copy of yesterday’s lecture notes with me?”

B: “I don’t have lah! I didn't even go for the lecture!”

Lah tends to have a sense of exasperation, it's a bit dismissive. "No lah, where got?!" It adds finality. It’s usually very confident.

A: “I’m really quite certain that I saw your husband buying 4D at 7–11!”

B: "No lah, cannot be! My husband is not that kind one lah!"

Often, novice Singlish speakers throw in Lahs into sentences where there isn't as much finality:

"Can I order some chicken rice lah?"

"Hey, I'm going for lunch lah!"

"Why you so like that lah?"

More appropriate usage might be:

"What am I getting for lunch? What else? Chicken rice lah!"

Lah can also be a little more mellow. "I don't really like him like that one lah…" But even in this mellow form, it's more resolute than Leh.