grammar - Future Simple vs. Future Perfect in difficult sentence

I would like to ask three questions:

1.) If the sentence mentioned below is gramatically correct.

Everything will be spotless by the time they get here.

2.) Is it correct to use Future Simple tense here? Or should it be Future Perfect as in sentence:

By the time we get there, everyone will have left. Nobody will even see you hair.

Also I would like to point one thing. First sentence describes a state ("be spotless"). The second describes an action ("left").

3.) Does this fact have an impact on using Future Simple (and not Future Perfect) tense in the first sentence?

Thank you very much in advance.

Answer

1.) Your examples seem grammatical in terms of tense (note at bottom on typo). Depending on context I might use a modal form (e.g. "Everything must be spotless") but I see no issue with using simple perfect.

2.) If you're asking whether you should use future perfect in the first example, note what that changes:

Everything will have been spotless by the time they get here.

In the first example, everything may not be spotless in the present but it will be by the time they arrive. In the future perfect version, everything may not be spotless in the present, but it will be for some period of time before the time they arrive. Future perfect is used to look backwards from some point in the future. That seems awkward unless you're emphasizing an earlier completion time. Here's that example with added context:

Guests are due to arrive at 6:30, but I will be done cleaning by 5. So everything will have been spotless by the time they get here.

Unless you specifically want to emphasize an earlier time, future perfect seems unnecessary here. This sentence would still convey the information that everything will be spotless in time:

Guests are due to arrive at 6:30, but I will be done cleaning by 5. So everything will be spotless by the time they get here.

The future simple signals the logical consequence of being done cleaning before the guests arrive.

3.) No, I don't see the difference between a state and an action affecting whether you should use future simple or future perfect tense. Instead it's functioning more on context and on whether you're emphasizing a future event or an event in the past from the perspective of the future.

(note) But check out "Nobody will even see you hair" in your second example. Do you mean "your hair?" "you there?" Something else?