How are the later Gurus considered reincarnations of the first when their lifetimes overlap? What is your personal understanding of concepts of soul, self or individuality, in light of this?
Later Gurus or Successors of Guru Nanak Dev Ji are not considered reincarnates of the first one. Reincarnation refers to the cycle of death and birth in which the soul changes physical body one after another. Guru is not stuck in this cycle. He is free from it and this is why he alone has the power to free us from this cycle. This is also why any human claiming to be a Guru is a hypocrite. When Sikhs say that all successive Gurus were Guru Nanak Dev Ji. it means that they were one spirit, thought and deed. The same Divine Light passed from one Guru to another. There was a time period when first four Gurus were physically present at the same time but only one was the true Guru. This is because only one had the divine light in him, not others. When this light is passed onto the Successor, he becomes the next Guru and the previous Guru is not the Guru anymore. The physical body is not the Guru but the Shabad(Gurbani) that is revealed. It is not the transfer of soul but transfer of divinity that united all of them.
The reason for choosing or appointing successors is very simple The Guru's mission was not only to preach the divine message but also to demonstrate it in practice and make sure it is ingrained in the community. Since it is not possible to show everything in practice in one lifetime, the mission was carried over by another. The Guru transferred his light to a successor who carried on the mission. When the Guru saw the near end of his mortal body, he appointed a successor upon which the Guruship was passed over officially to the Shabad and the collective body of the Khalsa. Under the guidance of the Shabad was given the right to make decisions, perform initiation ceremony and lead the community. This is why the concept of ‘Guru Granth, Guru Panth’ is prevalent in the community. Guru is present as a spirit in Gurbani and physically in the Five Beloved Ones.