“Upon attainment of arahandhood, that human being will live his life till the end of his remaining current human jati or after 7 days, parinibana?”
That holds ONLY for a layperson who has attained the Arahanthood.
– But a bhikku with Arahanthood will live to the end of his natural life. We know that Ven. Ananda lived for 120 years, even though he attained Arahanthood at a young age. The Buddha himself was an Arahant and lived for 45 years after the Buddhahood/Arahanthood.
If a layperson attains Arahanthood, he/she MUST become a bhikkhu/bhikkhuni within 7 days. That is because a layperson cannot “bear” the Arahanthood for more than 7 days.