Learn Portuguese (Portugal): Trata-se de perceber quão útil visualizar uma associação pode ser. - It is a matter of understanding how useful visualizing an association can be.

Yes, you can. In indirect degree clauses, quão and o quão are interchangeable when followed by an adjective (like útil) or an adverb. They both mean "how [adjective/adverb]". In spoken European Portuguese, o quão is often more common.

Why is the verb phrase pode ser pushed all the way to the end?

Clauses introduced by quão follow a specific word order: quão + adjective/adverb + subject + verb. In this sentence, the subject is the entire phrase visualizar uma associação (visualizing an association). The verb phrase pode ser must wait until after this long subject.

What is the function of trata-se de here?

Tratar-se de is a fixed impersonal expression meaning "to be a matter of" or "to be about". The pronoun se here simply makes the verb impersonal, acting like the English dummy subject "it" in "It is a matter of".

Why is "visualizing" translated as the infinitive visualizar rather than a gerund like visualizando?

In Portuguese, whenever an action functions as a noun—such as being the subject of the clause pode ser, as it is here—you must use the infinitive. English often uses the "-ing" form for this, but Portuguese uses the gerund almost exclusively for continuous, ongoing actions.