Rashi: Because 'The nations of the world', he told them, 'have many religions and many priests, who all ¹ gather in one house. We have only one G-d, one Aron, one Torah one Mizbe'ach and one Kohen Gadol - and you, two hundred and fifty men all want the Kehunah Gedolah! I want it too! So here's the most precious service of all - the Ketores ² , which is more treasured than all the Korbanos, and which contains poison ³ - the poison that killed Nadav and Avihu'. This was Moshe's warning to desist from going ahead with the test. He hoped that that they would do Teshuvah and relent. ⁴

Ramban #1 (on Pasuk 5) #1: It was his own idea.

Ramban (on Pasuk 5, citing the Ibn Ezra): Refer to 16:4:1:2.

Ramban (on Pasuk 5) #2: It came through Ru'ach ha'Kodesh ⁵ (Divine Inspiration).

Moshav Zekenim #1 (on Pasuk 7) #1: It was an excuse to delay until tomorrow (refer to 16:5:2:1), for it is offered only [half] in the morning and [half in the] afternoon, and presumably, one may begin only in the morning.

Moshav Zekenim #2 (on Pasuk 7) #2: Primarily, the Kohen Gadol offers it, even though a Drashah is Machshir even a Kohen Hedyot. ⁶

Rashi and Seforno: He was warning them that the chosen person was already Kadosh, that he was the one who would come out alive and all the others would die! ¹