He left Mina, as he was “hastening on”, but he did not do the farewell tawaaf until the following day. What is the ruling on that?

Firstly:

What matters with regard to “hastening on” is that the pilgrim should leave Mina before sunset on the twelfth (of Dhu’l-Hijjah). If he leaves at that time, then it does not matter after that if he delays the farewell tawaaf until the following day.

Secondly:

There is no problem, in sha Allah, with shaving the head before offering the sacrifice on the day of sacrifice, especially in the case of one who already did that in the past and is not looking for concession ahead of time.

Al-Bukhaari (124) narrated that ‘Abdullah ibn ‘Amr (may Allah be pleased with him) said: I saw the Prophet (blessings and peace of Allah be upon him) at the Jamrah as he was being asked questions. A man said: O Messenger of Allah, I offered the sacrifice before stoning the Jamrah. He said: “Stone it; it does not matter.” Another man said: I shaved my head before offering the sacrifice. He said: Offer the sacrifice; it does not matter.” He was not asked about anything that had been done sooner (ahead of its time) or later (delayed from its time) but he said: “Do it; it does not matter.”

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: One of the things that we learn from the verse – “and do not shave your heads until the Hady reaches the place of sacrifice” [al-Baqarah 2:196] – is that it is not permissible to shave the head until after offering a sacrifice, because Allah, may He be exalted, says, “until the Hady reaches the place of sacrifice”. This is the view of many of the scholars, who quoted as evidence the words of the Prophet (blessings and peace of Allah be upon him): “I have stuck my hair together (talbeed – whereby the pilgrim uses some sticky substance to stick his or her hair together in order to keep it orderly and protect against lice) and garlanded my sacrificial animal, so I will not exit ihram until I have offered my sacrifice.” Those who held this view based it on the apparent meaning of the verse, as well as the actions of the Messenger (blessings and peace of Allah be upon him), when he said: “so I will not exit ihram until I have offered my sacrifice.”

But there are several hadeeths which indicate that it is permissible to bring forward or delay these actions, so as to make things easier for the ummah. On the day of Eid the Prophet (blessings and peace of Allah be upon him) was asked about bringing some deeds forward and delaying others.  He was not asked about doing anything before or after something else but he (blessings and peace of Allah be upon him) said: “Do it; it does not matter.”

End quote from Tayseer al-Qur’an by Shaykh Ibn ‘Uthaymeen.

For more information please see the answer to question no. 106586

Based on that, your Hajj is valid, and you do not have to do anything, praise be to Allah.

And Allah knows best.