How much did Lombard know about the murders in "And Then There Where None"?

Question

When only Lombard and Vera are left alive, they have this dialogue that makes it seem like Lombard was the murderer. And while you could claim that it was a mutual misunderstanding (both of them thinking the other one was the murderer), there is this line that perplexes me.

Vera said:

"There's no one on the island - no one at all - except us two..." Her voice was a whisper - nothing more.

Lombard said: "Precisely. So we know where we are, don't we?"

Vera said: "How was it worked - that trick with the marble bear?"

He shrugged his shoulders. "A conjuring trick, my dear - a very good one..."

Here, it's as if Lombard admits he did all the murders, along with the last one. Which is obviously then proved to be a lie.

Before reading the epilogue, I was thinking that Morris had hired him as a murderer, so the line made sense. Which is again proven to not be the case. So, which one is it? How much did Lombard know about the plan, and why did he answer like that about the marble bear?

Answer 1

Lombard’s phrase “conjuring trick” refers back to chapter 14:

Lombard said: “Armstrong’s disappeared…”
Vera cried: “What?”
Lombard said: “Vanished clean off the island.”
Blore concurred: “Vanished—that’s the word! Like some damned conjuring trick.”

If Armstrong’s disappearance and Blore’s death were conjuring tricks, then who was the conjuror? Lombard now believes that it must have been Vera, since she is the only remaining suspect. However, he cannot work out how she did it, so when he says:

“A conjuring trick, my dear—a very good one…”

he means to compliment her on how completely she fooled everyone. But due to the ambiguous way he phrased the compliment, Vera interprets it as Lombard admitting his guilt.

(Ambiguous phrasing like this is one of Christie’s favourite tricks.)