Learn Hardcore Turkish: Üstelik toplantı odası doluydu, ayakta kaldık. - Moreover, the meeting room was full; we had to stand.

Ayrıca: neutral in addition; more formal, less emotional.
Hatta: even; can intensify surprise.
Hem de: even/and what’s more; colloquial emphasis. All can work, but Üstelik feels stronger and a bit more rhetorical than Ayrıca.

Why is it doluydu and not dolu oldu?

doluydu = was full (a state at that time).
dolu oldu = became full (a change into that state). Here the room was already full when you arrived, so doluydu is the natural choice.

How is doluydu formed morphologically?

From adjective dolu + past copula idi:

dolu + idi → doluydu (buffer y because dolu ends in a vowel). Vowel harmony applies: idi reduces to -ydi/-ydı/-ydu/-y dü depending on the last vowel. More examples:
yorgun + du → yorgundu
boş + tu → boştu

Could I use doluymuş or dolmuştu instead of doluydu?

Different nuances:

doluydu: simple past, direct knowledge.
doluymuş: hearsay/evidential past (you learned it indirectly).
dolmuştu: past perfect (it had become full before another past event). Use whichever matches the information source and timeline.

Is it okay to link the two clauses with a comma like that? Should I use ve or a cause connector?

Turkish often allows a comma between related clauses. Alternatives:

Coordination: ... doluydu ve ayakta kaldık.
Explicit cause: ... doluydu, bu yüzden ayakta kaldık. / Ayakta kaldık çünkü toplantı odası doluydu. The original comma implies a natural cause-effect without spelling it out.

Does ayakta kaldık literally mean “we stayed standing”? How is it different from ayakta durduk?

ayakta kalmak: remain standing (often because there were no seats). Idiomatic in crowded contexts.
ayakta durmak: stand/be standing (focus on the act, not the reason). To stress necessity: ayakta kalmak zorunda kaldık (we had to remain standing).

What is ayakta grammatically?

ayak + ta (locative case) = on one’s feet/standing. The locative uses -da/-de but becomes -ta/-te after a voiceless consonant (k in ayak), hence ayakta. Common fixed phrases: ayakta durmak, ayakta kalmak, ayağa kalkmak (stand up).

Why is the subject pronoun omitted? Where is biz?

Turkish conjugation encodes person/number. kaldık already means we stood. You add biz only for emphasis or contrast: Biz ayakta kaldık (we, as opposed to others).

What’s the structure of toplantı odası?

It’s a possessive compound (belirtisiz isim tamlaması):

toplantı (meeting) + oda (room) + 3rd sg possessive on the head noun → odası. No genitive on the first noun. Contrast:
toplantı odası = meeting room (type of room)
toplantının odası = the room of the meeting (specific possession)

Could I say toplantı salonu instead of toplantı odası?

Yes. salon suggests a larger hall; oda suggests a smaller room. Forms:

toplantı odası (meeting room)
toplantı salonu (meeting hall)

Can Üstelik be placed elsewhere in the sentence?

It’s most natural sentence-initial, but you can shift it to change focus:

Toplantı odası doluydu; üstelik ayakta kaldık. (what’s more, we had to stand) Placing it after the subject (Toplantı odası üstelik doluydu) is possible but marked; use sparingly.

What about the past ending in kaldık—why -dık and not -dik/-duk/-dük?

Past -DI obeys vowel harmony:

After a/ı → -dı/‑dık (here: kal‑ → kaldık)
After e/i → -di/‑dik
After o/u → -du/‑duk
After ö/ü → -dü/‑dük The consonant can surface as t after a voiceless consonant (e.g., koştu).

Could I make the causal link explicit with çünkü?

Yes: Ayakta kaldık çünkü toplantı odası doluydu. This makes the reason unmistakably clear.

Does Üstelik add a complaining tone here?

Often yes. Üstelik frequently appears in lists of inconveniences or surprises, so the sentence reads like an extra annoyance added to prior issues.