Imaam Ahmad bin Hanbal was asked a few questions regarding the friday prayer in the book Masa-il al-Imaam Ahmad bin Hanbal wa Is-haaq bin Rahaweyyah:

I asked him wether one person could give the khutbah and another person could lead the prayer

He replied: I do not know

I asked him if a person joins the Imaam in the last Raka'ah of the salah

He replied: Then he just adds one more Raka'ah, but if he joins the prayer whilst the people are in tashahud then he should pray four Raka'ahs

Is-haaq gave the same answer.

I asked him if there was any type of Imaam that the friday prayer would not be valid behind

He replied: No, the Friday prayer is not left for anything.

I asked him about the man who sneezes during the khutbah shall I say yarhamakAllah to him?

He replied: Yes do that

Is-haaq said the same thing

I asked him if I could reply to the greeting during the Khutbah

He replied: You should reply back

Is-haaq said: Yes

I asked him what if the place I am praying is very crowded and I cant preform ruku' nor sujud?

He replied: Then you make sajdah upon the back of the person infront of you.