Przez ten objazd autobus jest opóźniony.

Yes, przez very often means through, across, or via. But in this sentence it means because of or due to.

So:

• przez park = through the park
• przez ten objazd = because of this detour

This causal use of przez is quite common, especially in everyday speech.

Przez takes the accusative case, so yes, the noun phrase after it is in the accusative.

The reason it still looks like ten objazd is that for a masculine inanimate singular noun like objazd, the accusative has the same form as the nominative.

So:

• nominative: ten objazd
• accusative: ten objazd

Even though the form does not visibly change, the case is still accusative because przez requires it.

Objazd means a detour, diversion, or an alternative route used when the normal route is blocked or unavailable.

For example, if a road is closed and traffic has to go another way, that is an objazd.

So przez ten objazd suggests that the detour is causing the delay.

Yes, you absolutely can say:

Autobus jest opóźniony przez ten objazd.

Both versions are correct. The difference is mainly word order and emphasis.

• Przez ten objazd autobus jest opóźniony.
  Emphasizes the cause first: Because of this detour...

• Autobus jest opóźniony przez ten objazd.
  Starts with the bus and sounds a bit more neutral.

Polish word order is more flexible than English, so moving elements around often changes emphasis rather than basic meaning.

Jest opóźniony literally means is delayed.

Here, opóźniony is an adjective-like form derived from the verb opóźnić / opóźniać. In this sentence it functions like a predicate adjective after jest.

So:

• autobus jest opóźniony = the bus is delayed

This is a very natural way to say it in Polish.

You could also hear related expressions like:

• autobus ma opóźnienie = the bus has a delay
• autobus się opóźnia = the bus is getting delayed / is running late

But jest opóźniony is perfectly standard here.

This is a very common question.

• opóźniony = delayed
• spóźniony = late

For transport, schedules, trains, buses, flights, etc., opóźniony is usually the better choice.

Examples:

• Pociąg jest opóźniony. = The train is delayed.
• Lot jest opóźniony. = The flight is delayed.

Spóźniony is often used for people, or for something that arrives late in a more general sense:

• Jestem spóźniony. = I’m late.

So for a bus affected by a detour, opóźniony sounds more natural.

Ten can often correspond to this in English, but in real usage Polish does not always make the same this/that distinction as English.

So ten objazd most naturally means:

• this detour
• or that detour, depending on context

In many situations, Polish uses ten simply to point to a specific thing that is known in context.

If you removed it, the sentence would become more general:

• Przez objazd autobus jest opóźniony. = Because of a detour, the bus is delayed.

With ten, it sounds like a particular detour is being referred to.

No. There are several ways to express that idea.

Some common alternatives are:

• z powodu tego objazdu = because of this detour
• przez ten objazd = because of this detour
• w związku z tym objazdem = in connection with / due to this detour
• z powodu objazdu = because of the detour

Among these, przez ten objazd sounds very natural and conversational.

Because autobus is the subject of the sentence: it is the thing that is delayed.

In Polish, the subject is usually in the nominative:

• autobus = nominative singular

So the structure is basically:

• Przez ten objazd = because of this detour
• autobus = the bus
• jest opóźniony = is delayed

Yes. That is a very natural English translation.

Even though opóźniony is most directly delayed, English often uses late in this context, especially for buses and trains.

So depending on the style, these are all reasonable:

• Because of this detour, the bus is delayed.
• The bus is delayed because of this detour.
• The bus is late because of this detour.

The Polish sentence itself is perfectly normal for that meaning.