Learn Hardcore Norwegian: De pårørende venter i resepsjonen, men legen kommer snart. - The relatives are waiting at the reception, but the doctor is coming soon.

Yes. De is the 3rd person plural pronoun they. In this sentence it refers to the group de pårørende.
Be aware that Norwegian also has De as a formal/polite you, but here it’s clearly they because it’s followed by a noun phrase (pårørende).

What exactly does pårørende mean, and why is it used without an article?

Pårørende is a common term for relatives / next of kin / family members (of a patient). It’s often used in healthcare contexts.
It’s used here without an article because it’s part of a structure like De + noun meaning “They, the …” (a definite group known from context). You could also say De pårørende = “the relatives/next of kin”.

Is pårørende singular or plural here? It looks the same either way.

Here it’s plural because it refers to a group and matches De (they).
Pårørende can function as:

plural noun: pårørende (the relatives/next of kin)
or as an adjective-like word in some contexts (e.g., pårørende person is less common; more typical is en pårørende / de pårørende).

Why is it venter and not venter på?

Å vente can be used without an object when the meaning is simply “to wait” (as an activity): De venter i resepsjonen = “They are waiting in reception.”
If you specify what they’re waiting for, you usually use å vente på:

De venter på legen. = “They’re waiting for the doctor.”

What tense is venter and kommer?

Both are present tense (Norwegian present covers both “present” and many “near future” uses):

venter = “wait / are waiting”
kommer = “comes / is coming”

Does kommer snart mean “comes soon” or “is coming soon”?

Either is fine in English. Norwegian present tense often translates naturally as “is coming” for something imminent: legen kommer snart = “the doctor will be here soon / is coming soon.”

Why is it legen and not en lege?

legen is the definite form: the doctor. It implies a specific doctor known in the situation (e.g., the doctor responsible).
en lege would mean a doctor (not a specific one).

How is definiteness formed in Norwegian here (like legen)?

Norwegian commonly marks definiteness by adding a suffix to the noun:

lege = doctor
legen = the doctor
Similarly:
resepsjon = reception
resepsjonen = the reception (here: i resepsjonen)

Why does it say i resepsjonen (definite) instead of just i resepsjon?

With specific locations in a building (the reception desk/area of this place), Norwegian often uses the definite form: i resepsjonen = “in the reception (area).”
i resepsjon is unusual; you’d more often use the definite or a compound like i resepsjonsområdet (in the reception area).

What’s the function of men, and why is there a comma before it?

men means but and links two independent clauses:

De pårørende venter i resepsjonen
legen kommer snart
Norwegian normally uses a comma before coordinating conjunctions like men when they connect two full clauses.

Is the word order normal? Could it be different?

Yes, it’s standard SVO in both clauses:

De pårørende (subject) + venter (verb) + i resepsjonen (adverbial)
legen (subject) + kommer (verb) + snart (adverb)
You can move adverbs for emphasis, but you must keep the verb in 2nd position in main clauses (V2 rule), e.g. Snart kommer legen. (Soon the doctor is coming.)

How would you pronounce the tricky words pårørende and resepsjonen?

Approximate guidance (varies by dialect):

pårørende: POH-ruh-ren-duh (stress often on the first syllable pår-)
resepsjonen: reh-sep-SHUH-nən (stress around -sep-)
If you want, tell me whether you’re learning Bokmål with an Oslo/East Norwegian pronunciation focus, and I can be more specific.