Learn Hardcore Japanese: uriage ga hetta node, katyou ni soudansimasita. - Because sales decreased, I consulted with the section manager.

The particle が marks 売り上げ as the subject of the intransitive verb 減った (decreased). It simply introduces new information (“sales decreased”). If you used は instead, you’d be setting up 売り上げ as a topic and implying contrast or known context (“as for sales…”), rather than stating the fact that sales went down.

Why is the intransitive verb 減る used (as 減った) rather than the transitive 減らす?

減る is intransitive, meaning “to decrease on its own.” So 売り上げが減った means “sales went down by themselves.”
By contrast, 減らす is transitive (“to make something decrease”), and you’d need an agent marking (e.g. 売り上げを減らした would imply “we/they actively reduced sales”).

What’s the nuance difference between ～ので and ～から when giving reasons?

Both can mean “because/since,” but:

～から is more direct and neutral when stating a reason.
～ので is softer and more formal, presenting the reason as background information.
In a business context, 減ったので sounds polite and factual (“since the sales dropped…”), making the follow-on action (consulting) smoother.

Why is the clause before ので in the plain form (減った), while the main verb 相談しました is in the polite form?

Japanese often mixes plain form in subordinate clauses with polite ます/です in the main clause.
Here, ～ので requires the plain/past form (減った). The main action 相談しました remains polite to match business-level speech.

Why is 課長 marked with に in 課長に相談しました?

When you 相談する (consult/seek advice), the person you consult is marked with に.
So 課長に相談しました literally means “I consulted the section chief,” with に indicating the target or recipient of your consultation.

Could you say 課長と相談しました instead? What’s the difference?

Yes, but the nuance changes:

課長に相談しました focuses on you seeking advice from the section chief.
課長と相談しました emphasizes a mutual discussion with the section chief, as if you and they equally talked things through.

Why isn’t the topic of the consultation (the sales drop) explicitly marked as an object in the sentence?

Because the reason clause 売り上げが減ったので already states what you’re consulting about. In Japanese, if the content is clear, you can omit extra markers. If you wanted to be explicit, you could say:
売り上げが減ったことについて、課長に相談しました。
Here, こと + について (“about the fact that…”) spells out the topic, but it’s optional when context makes it obvious.