And hath been from his youth. Pray you, keep seat." She is lying and trying to convince the lords to stay and think that this is normal. In previous scenes Lady Macbeth was a strong woman who manipulated her husband into doing things she wanter to fulfill her ambition and dreams of becoming queen. In this scene she seems defeated and she is trying to cover up her husband's messes.

Who may I rather challenge for unkindness
Than pity for mischance!". This is dramatic irony because Macbeth knows why Banquo isn't at the banquet because he sent murderers to kill him but he is pretending that he is upset that Banquo isn't with them or that he is late. Macbeths court is telling Macbeth not to get mad that Banquo is late because Macbeth is just pretending to be upset but us as the readers/viewers/audience know that it is not the case because we know what Macbeth did.

Approach thou like the rugged Russian bear,

The armed rhinoceros, or th' Hyrcan tiger;

Take any shape but that, and my firm nerves

Shall never tremble. Or be alive again,

And dare me to the desert with thy sword.

If trembling I inhabit then, protest me

The baby of a girl. Hence, horrible shadow!

Unreal mockery, hence!'
This shows hs vulnerable side, which goes against the main quality of his, which is bravery, as he openly admits of being scared of Banquo's ghost, which I personally believe symbolizes both his murder and his sons, who will get the crown later in the play.
Hoever, as Macbeth starts a soliloquy, reminscing on his plan and actions, he understands that he is in too deep, an dthat there is no coming back, as he says 'All causes shall give way. I am in blood

Stepped in so far that, should I wade no more,

Returning were as tedious as go o'er.' meaning it would be now harder for him to go back to be the kind and brave Macbeth he used to be, and that he must now get his way around things by keeping to commit crimes and murders. This is then repeated and emphasized as he says, 'We are yet but young in deed.' as he believes he will keep committing crime after crime for a long time to come.

Ere humane statute purged the gentle weal;

Ay, and since too, murders have been performed

Too terrible for the ear. The time has been

That, when the brains were out, the man would die,

And there an end. But now they rise again

With twenty mortal murders on their crowns

And push us from our stools. This is more strange

Than such a murder is.' Here, Shakespeare uses imagery to show the horrible things done to Banquo, and how they affect MAcbeth's subconscious, as he then goes to speak to the witches the day after,

Hath nature that in time will venom breed;

No teeth for th' present. Get thee gone. Tomorrow

We’ll hear ourselves again.' This is emphasized with the use of alliteration as well.

As broad and general as the casing air.

But now I am cabined, cribbed, confined, bound in

To saucy doubts and fears.—But Banquo’s safe?'
The metaphor whole as a marble and founded as a rock, are comparisons to his throne, as they signify the setting in stone of his crown, and the metaphor of casing aur, showed how free of his guilts and paranoia he felt once Banquo was killed, as he even complimented the murderer. However, once he hears fleance escaped, he uses an oxymoron, paired up with the consonance of the sound c, that shows the constraints Macbeth feels now that he knows Fleance is alive.

Thy gory locks at me."
He fights with his subconscious, paranoid and afraid  of people finding him out even though unti lhe started imagining things, none of the lords even suspected a thing. 
This scene is very important in showing how much paranoia has affected his character development and drove the plot forward the way it did.

At the mention of banqou, macbeth sees the ghost, only he can see it, showing that it is only a figment of his imagination. He believes the ghost wants revenge, showing he recognises he has done wrong.

present; Who may I rather challenge for unkindness Than pity for mischance! this is as the audience understand macbeth can see banquo while the other people on stage cant, this is effective due to the fact that the people on stage thought he was insane or having a fit whereas the audience knew what was happening giving us an idea of how mad he has gotten.