Мы не нашли рычаг, не говоря уже о тайном ходе.

It means let alone, much less, or not to mention.

So:

• Мы не нашли рычаг = We didn’t find the lever
• не говоря уже о тайном ходе = let alone the secret passage

Literally, не говоря уже о is something like not even speaking of / not to mention, but in natural English let alone is often the best match.

Because не говоря уже о ... is a set parenthetical-style expression, and in Russian it is normally separated by a comma.

So the sentence is divided like this:

• Мы не нашли рычаг,
• не говоря уже о тайном ходе.

This is similar to English punctuation in phrases like:

• We didn’t find the lever, let alone the secret passage.

Because the expression не говоря уже о requires о + prepositional case.

That means:

• о ходе = about the passage
• о тайном ходе = about the secret passage

So both words change:

• тайный → тайном
• ход → ходе

This is just normal case government after о in this expression.

It is in the accusative, but for an inanimate masculine noun like рычаг, the accusative singular looks the same as the nominative singular.

So:

• nominative: рычаг
• accusative: рычаг

Since найти takes a direct object, рычаг here is accusative.

Good question. Russian can sometimes use the genitive under negation, so не нашли рычага is possible in some contexts.

But in modern standard Russian, the accusative is also very common after negation, especially when the object is seen as a specific, concrete thing:

• не нашли рычаг = didn’t find the lever

Using рычага can sound a bit more like:

• didn’t find any lever
• or it may feel slightly more bookish or stylistically marked depending on context

So не нашли рычаг is perfectly normal.

Because нашли is the past plural perfective form of найти (to find), and perfective is used for a completed result.

Here the idea is:

• they tried to find it
• the result was negative
• they did not find it

So не нашли means didn’t find in the sense of failed to find.

If you used не находили, it would usually suggest repeated or ongoing action, or a different kind of context, such as were not finding or had not been finding.

Because Russian past tense agrees with the subject in gender/number, not person.

The subject is мы = we, so the verb must be plural past:

• я нашёл / нашла
• ты нашёл / нашла
• он нашёл
• она нашла
• мы нашли
• они нашли

So нашли is simply the correct past plural form.

Рычаг means lever.

It can mean:

• a literal mechanical lever
• sometimes a figurative means of influence or tool of control

In this sentence, it almost certainly means a physical lever, because it is being contrasted with тайный ход (secret passage), which sounds like a physical setting such as a puzzle, mechanism, or hidden entrance.

Here тайный ход means secret passage, hidden passage, or secret way in/out.

The noun ход has several meanings in Russian, including:

• movement
• course/progress
• turn in a game
• passage/way

In this context, with тайный, it means a hidden route or passage, not a move in a game.

So:

• тайный ход = secret passage

Yes. It is a very common fixed expression.

Pattern:

• X, не говоря уже о Y = X, let alone Y

Examples:

• Он не может купить кофе, не говоря уже о машине.
  He can’t afford coffee, let alone a car.

• Я не понимаю эту статью, не говоря уже о всей книге.
  I don’t understand this article, let alone the whole book.

It usually introduces something that is even less likely, harder, or more extreme than the first thing mentioned.

Yes, Russian word order is flexible, though the original version is the most neutral.

Original:

• Мы не нашли рычаг, не говоря уже о тайном ходе.

You could also say:

• Рычаг мы не нашли, не говоря уже о тайном ходе.

This puts extra focus on рычаг.

But for a learner, the original order is the safest and most natural default.