Parshas Tzav - When was Moshe a cohen?

The passuk says in this week’s sedrah (8:28)

וַיִּקַּח מֹשֶׁה אֹתָם מֵעַל כַּפֵּיהֶם וַיַּקְטֵר הַמִּזְבֵּחָה עַל הָעֹלָה מִלֻּאִים הֵם לְרֵיחַ נִיחֹחַ אִשֶּׁה הוּא לַה'

And Moshe took the parts of the איל המלואים and the matzos from the hands of Aharon and his sons and he burnt them on the mizbeach.

Rashi comments

ויקטר המזבחה - משה שמש כל שבעת ימי המלואים בחלוק לבן

Moshe served as a cohen all seven days of inauguration in a white robe (which he owned himself and which was not a part of the בגדי כהונה).

• Why did Rashi only explain that Moshe served as the cohen in the inauguration of the cohanim and the mishkan on this passuk, why did Rashi not explain that Moshe assumed the function of a cohen, in order to perform the inauguration, at the beginning of the parsha?

The Maharal (Gur Aryeh ibid.) begins his answer to this question by pointing out that it is not obvious that Moshe did indeed perform the עבודה for all seven days.

• Maybe Moshe only did the עבודה on the first day, and once he had shown Aharon and his sons how the עבודה was performed, then Aharon his sons did the עבודה the other six days. How does Rashi in fact know that Moshe did the עבודה for all seven days?

The Maharal explains that Rashi is answering both questions by addressing answering a hidden question on this passuk.

Rashi previously explained (7:30) that three cohanim are required to do the avodah of a קרבן תנופה. One cohen brings the parts of the korban from where it has been cut up, one cohen places the parts on the hands of the person bringing the korban and waves the parts together with him, and one cohen  then takes the parts of the korban and burns them on the mizbeach. The reason that one cohen does not perform all three עבודות is because of the rule of ברוב עם הדרת מלך. It shows greater respect for Hashem if three cohanim all do a part of the עבודה than if one cohen does the entire עבודה himself.

If so, why did Moshe perform all three עבודות of the תנופה of the איל המלואים and the matzos himself, why did he not ask two of the other cohanim to help him so that the עבודה would be done by three people?

Rather you see from this that it was not possible for Moshe to ask Aharon and his sons to help him in the עבודה because only Moshe was qualified to do the עבודה during the שבעת ימי המלואים. Since we see from this passuk  that only Moshe could do the עבודה, therefore it must be that Moshe did the עבודה for all seven days, and not that he trained in Aharon and his sons on the first day.