What is a "fir tree"?

Can I say "we've been dying out here" instead of "we're dying out here"?

Yes, you can. "We've been dying out here" and "we're dying out here" technically express the same meaning, however the latter is used more commonly as an expression. There are different variations of this expression you can use: Ex: I'm dying of thirst. Can I get some water? Ex: I'm dying in here! Turn on the air conditioner ASAP please! Ex: We're dying out here! Can we please come inside?

Does "news" have plural form?

"News" only has one form, which is considered singular and uncountable. Thus, there is no plural form. The verbs you use with the word "news" must also correspond to the singular form as well. Ex: The news was shocking. Ex: The bad news is that our car broke down. The good news is that Jack fixed it.

What's the difference in nuance if I say "keeps" instead of "lasts" here?

"Last" and "keep" as verbs both mean to "continue on" or "endure", however, they have very different nuances. To "last" just means to continue on for a specific period of time while "keep" is usually only used in the context where something will no longer be able to work after a certain amount of time and will go bad afterwards, such as food. Ex: This food will keep for a week in the fridge. Ex: My cough lasted for a few days. Ex: How long is this movie going to last?

What does the word "shimmer" mean? In what circumstances could I use it?

To "shimmer" means to shine with a soft light or glisten. Here, she's saying that she's sparkly and will light up a place with actual jewels or metaphorically from her personality and energy. Ex: I like how diamonds shimmer in the sunlight. Ex: Your dress is shimmering with all the sequins on it. Ex: The water is shimmering.

When can I use the expression "I'll ever be"?

"I'll ever be" is not usually its own expression. The full expression here, "ready as I'll ever be", means that someone may not be fully ready, but they won't be any more ready later on. In other words, they are already as prepared for something as they ever could be able to be prepared. This expression is often used when someone does not feel entirely confident that they are prepared. Ex: "Are you ready for the exam?" "Ready as I'll ever be."