Für den Fall, dass die Dichtheitsprüfung scheitert, kommt der Fachbetrieb.

Für den Fall, dass means something like in the event that or if it should happen that.

It is a bit more formal and more explicit than wenn or falls.

• Für den Fall, dass die Dichtheitsprüfung scheitert, ... = In the event that the leak test fails, ...
• Wenn die Dichtheitsprüfung scheitert, ... = If the leak test fails, ...
• Falls die Dichtheitsprüfung scheitert, ... = If / in case the leak test fails, ...

So the sentence could be rewritten more simply as:

• Falls die Dichtheitsprüfung scheitert, kommt der Fachbetrieb.

The original version sounds a little more official or written.

Because the preposition für always takes the accusative case.

So:

• der Fall = nominative
• den Fall = accusative

That is why the sentence says:

• Für den Fall, dass ...

not

• Für der Fall, dass ...

This is just normal case government after für.

Because dass introduces a subordinate clause, and in German subordinate clauses are separated by a comma.

Here the subordinate clause is:

• dass die Dichtheitsprüfung scheitert

So the structure is:

• Für den Fall
  • , dass die Dichtheitsprüfung scheitert,
    • kommt der Fachbetrieb.

German uses commas more consistently than English for this kind of clause.

Because dass introduces a subordinate clause, and in German the finite verb goes to the end of a subordinate clause.

So:

• dass die Dichtheitsprüfung scheitert

Compare:

• Main clause: Die Dichtheitsprüfung scheitert.
• Subordinate clause: ..., dass die Dichtheitsprüfung scheitert.

This is one of the most important German word-order rules:

• main clause: verb usually in second position
• subordinate clause: verb usually at the end

Because the sentence begins with a fronted element:

• Für den Fall, dass die Dichtheitsprüfung scheitert

In a German main clause, the finite verb must come in second position. Since the whole opening phrase takes the first position, the verb kommt has to come next.

So the structure is:

1. Für den Fall, dass die Dichtheitsprüfung scheitert
2. kommt
3. der Fachbetrieb

If you put the subject first, you would get:

• Der Fachbetrieb kommt, falls die Dichtheitsprüfung scheitert.

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

Dichtheitsprüfung is a compound noun, which is very common in German.

It is made up of:

• Dichtheit = tightness, leakproofness
• Prüfung = test, inspection, examination

So Dichtheitsprüfung is a leak test, tightness test, or leakproofness inspection, depending on context.

A useful strategy with German compounds is to read them from the end:

• Prüfung tells you it is some kind of test
• Dichtheits- tells you what kind of test it is

So: a test of tightness / leakproofness.

Here scheitern means to fail or to be unsuccessful.

So:

• Die Dichtheitsprüfung scheitert = The leak test fails

A useful nuance:

• durchfallen is often used for failing an exam
• scheitern is broader and can be used for attempts, plans, negotiations, projects, or tests that do not succeed

Examples:

• Der Plan scheitert. = The plan fails.
• Die Verhandlungen scheitern. = The negotiations fail.
• Die Prüfung scheitert is understandable in a technical context like this.

Fachbetrieb means a specialist business/company or qualified professional firm.

It usually refers to a company with professional expertise in a particular trade, for example plumbing, electrical work, heating, roofing, and so on.

In this sentence, der Fachbetrieb suggests that a qualified specialist company will come if the test fails.

So it is not just any business; it implies expertise and proper professional standards.

Yes. Common alternatives are:

• Falls die Dichtheitsprüfung scheitert, kommt der Fachbetrieb.
• Wenn die Dichtheitsprüfung scheitert, kommt der Fachbetrieb.

These are more everyday and less formal.

A rough comparison:

• wenn = neutral, very common
• falls = a bit more like in case / if it happens that
• für den Fall, dass = more formal, more written, more explicit

Yes. If the main clause comes first, that clause follows normal main-clause word order:

• Der Fachbetrieb kommt, falls die Dichtheitsprüfung scheitert.

Here:

• Der Fachbetrieb = subject in first position
• kommt = verb in second position

The subordinate clause still keeps its verb at the end:

• falls die Dichtheitsprüfung scheitert

So only the main-clause order changes depending on what you place first.

With Für den Fall in this structure, yes:

• Für den Fall, dass die Dichtheitsprüfung scheitert, ...

The phrase normally works as a fixed expression: für den Fall, dass ...

You cannot normally say:

• Für den Fall die Dichtheitsprüfung scheitert

That would be incorrect.

If you want to avoid dass, you would usually switch to another conjunction such as falls or wenn:

• Falls die Dichtheitsprüfung scheitert, ...
• Wenn die Dichtheitsprüfung scheitert, ...

Because the subordinate clause is inserted before the main clause, and it has to be closed off with a comma.

So the sentence has this pattern:

• Für den Fall, dass die Dichtheitsprüfung scheitert, kommt der Fachbetrieb.

The subordinate clause begins with dass and ends after scheitert, so commas mark its boundaries.

This is very standard German punctuation when a subordinate clause appears before the main clause.