Hi! Thanks for your question.

The main answer to your questions is time - Tutankhamen died in circumstances where nobody expected him to die. The tomb simply wasn't ready for him - as he'd come to the throne only a few years previously, work had really only just begun on his tomb. Therefore, the tomb he was actually found in (KV62) was not the one that was meant for him.

This is indicated by the size of the tomb, for starters. The burial chamber that the sarcophagus was found in was incredibly small considering its contents. Tutankhamen was buried in 3 anthropoid coffins, surrounded by a box coffin, surrounded by 4 shrines. The largest of these shrines, for want of a better word, was massive. It filled the burial chamber, leaving little more than a foot of space around it. In order to get the shrines into the tomb, the entire sarcophagal structure would have been deconstructed and then put back together within the chamber. It was not practical or usual for there to be such a lack of space in a royal tomb such as this - it would have been a very difficult process considering the extent of the funerary equipment. Therefore it suggests that there were no other options - if there had been a larger tomb ready for Tutankhamen in the Valley of the Kings, it would have been used. However, its very unlikely that his personal tomb would be ready for him.

The rest of the tomb was similarly packed - approximately 700 artefacts, all crammed into the small antechamber. While its likely that tomb robberies would have disturbed the material, it was still unusual to see so much funerary equipment in such a small space, particularly in a royal tomb, where there would normally be more space to accommodate the extensive material.

When it comes to the paint drying issues, this is one of the main indicators that King Tut was buried in a hurry. Once the pharaoh dies, there's only a specific amount of time before he needs to be buried. The mummy is embalmed for 40 days, and after that there is only a small window in which funeral rites must be performed and the king buried. As a result, the workers needed to get the decoration on the chamber done quickly. Egyptian art is extremely precise, with very specific conventions and measurements. Therefore it would have been a long process, made more difficult by the fact that they had very little time.

Furthermore, the nature of the decoration is unusual for a royal tomb. Normally in the New Kingdom we would expect to see the Netherworld Books painted all over the walls of the tomb, but in the case of KV62, the decoration is confined to the burial chamber only. This, along with the paint marks, suggest that the chamber was painted in a hurry, and as it was the burial chamber, was considered the most important and therefore needed to be painted, while time constraints meant that they didn't have time to finish the rest of the chambers. Riggs (2018) has some good photography on this.

As you mentioned, on the famous burial mask of Tutankhamen, there is evidence of the name Ankhkheperure mery-Neferkheperure written on it, which was the royal name of Nefertiti, Akhenaten's chief wife. Reeves (2003) notes that this is likely due to the swiftness of Tutankhamen's death - there was likely no burial mask made for him at the time and therefore another mask, that of Nefertiti, was modified (as you said regarding evidence of facial changes in the mask) and used instead. The anthropoid coffins also suggest that they did not originally belong to Tutankhamen - the features are much more angular and seem older, leading Egyptologists to believe they may have originally belonged to Smenkhare, who is likely the mummy from KV55.

Now, as I've mentioned, the main reason for all of these unusual aspects of the tomb is the swiftness of the King's death. It was originally thought that Tutankhamen died from a broken leg as the result of a chariot accident - this was a logical theory as the leg was indeed broken on CT scans, and chariots were found in his tomb. However, more recent studies have concluded that while Tutankhamen died from a broken leg, he already had a genetic disorder which caused physical disabilities. This unexpected death would have caused chaos in terms of his burial - he had only been king for a few years and therefore it was likely that plans for his tomb had only just begun. Pharaohs plan for their death years in advance, but to design, carve out and decorate a tomb, particularly one fit for a king, takes decades. It was simply not possible to create a tomb appropriate for him in such a short time. Therefore, the most available tomb was used. He also likely didn't have much funerary equipment, considering that he had a short reign and was very young. As a result, funerary objects from other royals were used - Nefertiti, and Smenkhare, who was likely the brother of Tutankhamen.

Bibliography:

Booth, Charlotte (2007). The Boy Behind the Mask: Meeting the Real Tutankhamun. Oneworld.

Connolly, B., & Godenho, G. (2015). Investigating Tutankhamun: Comparing CT Scans and X-Rays. In P. N. Campbell Price, R. Morkot, J. Tyldesley, A. Chamberlain, & R. Forshaw (Eds.), Manchester: Manchester University Press.

Hawass, Zahi (2004). The Golden Age of Tutankhamun. American Univ in Cairo Press.

Reeves, Nicholas (2003). "THE GOLD MASK OF ANKHKHEPERURE NEFERNEFERUATEN". Journal of Ancient Egyptian Interconnections, Vol. 7, No. 4.

Riggs, Christina (2018). Photographing Tutankhamun: Archaeology, Ancient Egypt, and the Archive. Bloomsbury Publishing.