Act 3 Scene 6 Lennox seems to be very careful in his speech. Why is he cautious in what he says? What, if anything, could be interpreted as a...

Lennox IS quite careful in his speech. In Act 3, scene 5,
Macbeth tells his wife that he has spies in every lord's
house:

There's
not a one of them but in his house

I keep a servant
fee'd.

Consequently, it pays
to be cautious in criticizing the king, especially when the consequences can be severe.
 Macbeth has turned against Macduff not only because of the apparition's prophesy, but
also because his spies have discovered Macduff's disloyalty.  In this speech, Lennox is
ironically critical of Macbeth.

His words taken at face
value can go for praise. He says in reference to Macbeth's killing Duncan's
guards:

Did
he [Macbeth] not straight,

In pious rage, the two
delinquents tear,

That were the slaves of drink and thralls
of sleep?

Was that not nobly done?  Ay, and wisely
too;

For 'twould have angered any heart
alive

To hear the men
deny't.

A spy reporting
Lennox's words might have a difficult time proving that Lennox is being critical at all.
 Yet, Lennox's irony is far from subtle.  Within his speech, Lennox implies that it is
ludicrous to believe that Duncan's sons killed their father.  He suggests in a darkly
humorous way that Macbeth will probably try to pin the murder of Banquo on Fleance,
because he, like Donalbain and Malcolm, fled.  Lennox predicts that Macbeth would kill
Donalbain, Malcolm, and Fleance if they were in his custody.  He implies that killing
Duncan's guards was a wise act for Macbeth because they would have denied taking any
part in the killing of Duncan--why?  because they were
innocent!

At the end of this scene, though, Lennox drops
his irony and sarcasm.  He more directly refers to Macbeth as a "hand accursed" that is
ruling "our suffering country."  He wants Macduff to exercise caution and prays that
Macduff will be able to get help from England to overthrow
Macbeth.