Nichiren bestowed more than one Gohonzon on several individuals. Don't worry. If you like the Gohonzon for the Transmission of the Dharma or the Shutei Gohonzon, and wish to chant to more than one Nichiren Gohonzon from time to time, go ahead.
Did you know that Kyo'o Gozen died? So, if the infant daughter of the perfectly faithful Shijo Kingo died prematurely, a similar tragedy could befall any one of us. Never let anything destroy your faith. Birth, old age, illness, and death is one aspect of "immutable karma". Likewise, eternal lifespan is another aspect of "immutable karma" (thanks to the process of reincarnation). Nichiren teaches us that, when the practitioner attains Buddhahood, one's ancestors and descendants, for seven generations back and seven generations forward, attain Buddhahood. This is why we chant ceaselessly to attain Buddhahood and cause others to do the same.