Responses

Indeed. When Wooju went to his apartment and Professor Cha proposed sex and said he had a partner on the way, I assumed it was Beta, because he himself said that if he was attracted to Beta, it was just sexual desire. If he wanted to sleep with Omega, it would be reproductive desire (something like that). That's why I assumed the new person was Beta.
I don't know why the beta that Professor Cha is sleeping with brought him to his apartment. Especially if he knew about his heat (and he must have known, probably the professor himself informed him about it and wanted him to come help him relieve it). Were they counting on a threesome?