Habida cuenta de que hay riesgo, actuamos.

It is a fixed formal connector. It introduces a reason or circumstance and is used a bit like:

• given that
• in view of the fact that
• taking into account that

In modern Spanish, learners usually treat it as one chunk: habida cuenta de que + clause.

It is especially common in formal writing, such as legal, administrative, academic, or journalistic Spanish.

Because this expression is a frozen old-style structure in which habida agrees with cuenta, which is a feminine singular noun.

So the phrase is fixed as:

• habida cuenta de...

You should learn it as a set expression rather than trying to build it freely yourself. In normal modern Spanish, you would not usually create similar phrases productively.

Because the full expression is habida cuenta de. The preposition de belongs to the expression itself.

So:

• habida cuenta de + noun phrase
  Habida cuenta del riesgo...
• habida cuenta de que + clause
  Habida cuenta de que hay riesgo...

This is not an error. It is not unnecessary de. The phrase simply requires it.

Not very. It sounds formal and somewhat bureaucratic/literary.

In everyday Spanish, especially in conversation, people are more likely to say things like:

• como hay riesgo...
• ya que hay riesgo...
• dado que hay riesgo...
• teniendo en cuenta que hay riesgo...

So yes, the sentence is correct, but it is not especially colloquial.

Because the speaker is presenting the risk as a real, accepted fact, not as something doubtful, hypothetical, or desired.

That is why indicative is used:

• hay riesgo

The expression habida cuenta de que normally introduces information treated as factual, so the indicative is the natural choice here.

Because Spanish often omits subject pronouns when the verb ending already makes the subject clear.

• actuamos = we act / we acted

The ending -amos tells you the subject is nosotros.

You could add nosotros for emphasis or contrast:

• Nosotros actuamos...

But in a neutral sentence, leaving it out is more natural.

In isolation, actuamos can be ambiguous:

• present: we act
• preterite: we acted

In this sentence, the presence of hay usually pushes the interpretation toward the present:

• Habida cuenta de que hay riesgo, actuamos.
  = Given that there is risk, we act.

If the sentence were in a past context, Spanish would normally make that clearer, for example:

• Habida cuenta de que había riesgo, actuamos.

So grammatically actuamos can be either, but here present is the most likely reading.

Because the sentence begins with a fronted introductory expression:

• Habida cuenta de que hay riesgo, ...

That first part sets the circumstance or reason, and the comma marks the pause before the main clause:

• actuamos

In formal writing, this comma is very natural and recommended.

Because hay riesgo is a common way to speak about risk in a general, uncounted sense.

Compare:

• hay riesgo = there is risk / there is danger
• hay un riesgo = there is a specific risk

In your sentence, the general idea is enough, so hay riesgo sounds more natural.

Yes. If you want something less formal, these are very natural alternatives:

• Dado que hay riesgo, actuamos.
• Como hay riesgo, actuamos.
• Ya que hay riesgo, actuamos.
• Teniendo en cuenta que hay riesgo, actuamos.

Among these:

• como, ya que = common and natural
• dado que = formal but still common
• teniendo en cuenta que = clear and widely used
• habida cuenta de que = the most formal/literary of the group

It is understood across the Spanish-speaking world. It is not exclusive to Spain.

That said, its formal tone is the important point. Whether in Spain or elsewhere, it tends to sound more at home in:

• official writing
• legal texts
• formal argumentation
• academic prose

So the issue is less regional and more register.