Allah describes the rebellion and stubbornness of the idolaters in defying the truth and arguing against it,
{And even if We had sent down unto you a Message written on paper so that they could touch it with their hands,}

meaning, if they saw this Message's descent and were eye-witnesses to that,
{the disbelievers would have said: "This is nothing but obvious magic!"}

This is similar to Allah's description of the disbelievers' defiance of facts and truth,
{And even if We opened to them a gate from the heaven and they were to continue ascending thereto. They would surely say: "Our eyes have been (as if) dazzled. Nay, we are a people bewitched."} [15:14-15], and,
{And if they were to see a piece of the heaven falling down, they would say, "Clouds gathered in heaps!"} [52:44].
{And they say: "Why has not an angel been sent down to him?"}

to convey the Message with admonition along with him. Allah replied,
{Had We sent down an angel, the matter would have been judged at once, and no respite would be granted to them.}

Consequently, even if the angels descend, while the disbelievers still had the same attitude, then the torment will surely befall them from Allah as a consequence. Allah said in other Ayat,
{We send not the angels down except with the truth (i.e. for torment, etc.), and in that case, they (the disbelievers) would have no respite!} [15:8], and,
{On the Day they will see the angels, no glad tidings will there be for the criminals that day.} [25:22]
Allah's statement,
{And had We appointed him an angel, We indeed would have made him a man, and We would have certainly caused them confusion in a matter which they have already covered with confusion.}

meaning, if We send an angel along with the human Messenger, or if We send an angel as a Messenger to mankind, he would be in the shape of a man so that they would be able to speak to him and benefit from his teachings. In this case, the angel (in the shape of a human) will also cause confusion for them, just as the confusion they caused themselves over accepting humans as Messengers! Allah said,
{Say: "If there were on the earth, angels walking about in peace and security, We should certainly have sent down for them from the heaven an angel as a Messenger."} [17:95]
It is a mercy from Allah to His creation that He sends every type of creation, Messengers from among their kind, so that they are able to call their people to Allah, and their people able to talk to them, ask them and benefit from them. In another Ayah, Allah said;
{Indeed Allah conferred a great favor on the believers when He sent among them a Messenger from among themselves, reciting unto them His verses (the Qur'an), and purifying them.} [3:164]
Ad-Dahhak said that Ibn 'Abbas said about the Ayah [6:9 above], "If an angel was sent to them, he would come in the shape of a man. This is because they will not be able to look at the angel due to light."
{...and We would have certainly caused them confusion in a matter which they have already covered with confusion.}

meaning, We would confuse them over their confusion. And Al-Walibi reported Ibn 'Abbas saying; "We brought doubts around them."
Allah's statement,
{And indeed Messengers were mocked before you, but their scoffers were surrounded by the very thing that they used to mock at.}

comforts the Messenger concerning the denial of him by his people. The Ayah also promises the Messenger, and his believers, of Allah's victory and the good end in this life and the Hereafter. Allah said next,
{Say: "Travel in the land and see what was the end of those who rejected truth."}

meaning, contemplate about yourselves and think about the afflictions Allah struck the previous nations with, those who defied His Messengers and denied them. Allah sent torment, afflictions and punishment on them in this life, as well as the painful torment in the Hereafter, while saving His Messengers and believing servants.