Is it true that a Muhaddith (Hadith scholar) may sometimes narrate about himself, or that his students transmit certain narrations from him, such as the example mentioned in Sahih al-Bukhari concerning Muhammad and Saeed bin Abi Maryam?

There is no such term as "a muhaddith (hadith (definition) scholar) updating from himself." What appears in hadith books as the compiler's name preceded by performance formulas is from the statement of the narrator transmitting from him.

Thus, in Musnad Ibn Rahawayh, the narrator transmitting from him is Imam Abdullah ibn Muhammad ibn Shirawayh. And what begins with the phrase "Imam Abu Ya'qub Ishaq ibn Ibrahim al-Hanzali informed us" or "Imam Abu Ya'qub Ishaq ibn Ibrahim al-Hanzali said" is from the words of Ibn Shirawayh.

As for the example from Sahih (definition) al-Bukhari in the chapter "Mention of the Angels" with the phrase: "Muhammad narrated to us, Ibn Abi Maryam narrated to us...", scholars have differed in identifying this "Muhammad." Some preferred that it was Al-Bukhari himself, while others preferred that it was Muhammad ibn Yahya al-Dhuhli, which Ibn Hajar strengthened elsewhere in "Hady al-Sari," where he pointed to the omission of "Muhammad" in some narrations and to Al-Hakim's preference for it being Al-Dhuhli.

The mention of the compiler's name explicitly in Sahih al-Bukhari has occurred in the words of Al-Firabri, such as: "Muhammad ibn Yusuf al-Firabri informed us, and Muhammad ibn Isma'il al-Bukhari narrated to us...", and his statement: "Muhammad ibn Yusuf al-Firabri said: It was mentioned from Abu Abdullah, from Qabisah...", where "Abu Abdullah" is Al-Bukhari himself.