Basically, you already have understand the main points. To sum it up, the Rome's Comitium was rectangular until around 300 BC when it got circular stands/bleachers. Sadly, the stratigraphical record is difficult to interpret and the Comitium has been destroyed and rebuilt by Sylla in the 1st century BC. So the historians rely on the changes occurring in the Roman colonies to get hints about the date of its changes. Circular Comitium in colonized cities give a range of possible dates for the adoption of this architectural characteristic, like Cosa (273 BC), Paestum (273 BC), Alba Fucens (303 BC) and Fregellae (328 BC although debated). The author of the linked chapter suggests a narrow range by including the moment when Pythagorean ideas reach Rome and the tribes reforms by Appius Claudius in 312 BC.
The two pictures you choose are the Comitium of Cosa and of Paestum. Here, from another publications, the Comitium from Alba Fucens and Fregellae.
However, the historical representations of the second century BC Comitium in Rome, is a bit simpler:
The Rome's Comitium is associated with the Graecostasis, Columna Maenia, Rostra Vetera and the lapis niger. Statues of Pythagore and Alcibiade are also mentioned.