Learn Portuguese (Portugal): Será que o acidente na rotunda foi caótico? - Was the accident in the roundabout chaotic?

Adding será que (literally "will it be that") introduces a nuance of speculation or wonder. It translates closer to "I wonder if the accident was chaotic?" or "Do you suppose the accident was chaotic?" rather than just demanding a direct yes-or-no fact.

Why do we use foi (from the verb ser) here instead of esteve (from estar), since the chaos was temporary?

In Portuguese, when describing an event or an occurrence (like a party, a meeting, or an acidente), you always use ser, not estar. Even though the chaos didn't last forever, you are describing the fundamental nature of how that event unfolded, rather than a temporary physical state.

How would I answer this question using de tal modo que to say it was so chaotic that the traffic stopped?

You could say: O acidente foi de tal modo caótico que o trânsito parou. Because you are stating a factual outcome of the intensity (the traffic actually stopped as a result), the verb after que stays in the indicative mood (parou).

Is rotunda the universal Portuguese word for "roundabout"?

In Portugal, rotunda is the standard, everyday word for a roundabout. However, it's worth noting as a B2 learner that this is specific to European Portuguese; Brazilian Portuguese typically uses the word rotatória.