Genesis block maybe defined as the first block that we obtain after the creation of any blockchain as well as if I talk about importance of Genesis block then all the subsequent blocks that creates after Genesis block follow the information of Genesis block as well as you have also give an us important information about block ID, timestamp as well as about witness signature

I agree with you the reasons that you providing for down time that when there is system upgrade or when there is any disturbance in hardware infrastructure then it could be down time of a blockchain and their is no need to be worry at that time because it only occurs for a temporary period of time for some hours or 10 to 12 hours I wish you good luck in your entry because you have good information about the topic as you answer all the questions according to requirement