Do you think Lady Macbeth would have acted sooner to cover Macbeth's suspicious comments at the banquet table if she'd known of Macbeth's plan...

Lady Macbeth is a smart cookie.  In III, ii, Macbeth says
to her:

Thou
know'st that Banquo, and his Fleance, lives....

There's
comfort yet.  They are
assailable.

and

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...there shall be done a deed of dreadful
note.

So, though Macbeth
tells Lady M to "Be innocent of the knowledge," it isn't impossible to imagine that she
gets his drift about killing them.  And in III, iv, when the murderer appears at the
door with blood on his face, it isn't too far-fetched to imagine that, if she didn't
know before what has happened, that she does now.

The thing
that she doesn't get right away is that Macbeth sees Banquo's ghost.  This, potentially,
isn't necessarily because she doesn't know that the murders are being committed, but
that Banquo's ghost is invisible to everyone but Macbeth, so she doesn't know exactly
what it is that he's reacting to.

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Ross

...Please't
your highness

To grace us with your royal
company.

Macbeth

The
table's
full.

Lennox

Here's
a place reserved,
sir.

Macbeth

Where?

Lennox

Here,
my good lord.  What is't that moves your
highness?

Macbeth

Thou
canst not say I did it: never shake

Thy gory locks at
me.

Ross

Gentlemen
rise; his highness is not well.

Lady
Macbeth

Sit worthy friends.  My lord is
often thus.

She seems right
on top of it when it happens, but her cover is not exactly very convincing, maybe
because Macbeth's behaviour is so telling that it is nearly impossible to "cover it." 
Until he mentions "shaking thy gory locks," she doesn't realize what's happening because
his comments about the table being full are merely puzzling, not revealing of what he
sees.

So, I wouldn't be so quick to assume that Lady
Macbeth doesn't get Macbeth's drift about murdering Banquo, and I also wouldn't be so
sure that there would have been any time "sooner" to react to what was going on in the
banquet.