This sounds risky based on current rules. There are currently no scheduled commercial flights between the two countries. What airline is telling you otherwise?
Why don't you get a rapid PCR covid test in Arusha and have a driver take you to Nairobi in time for your flight? You will need to switch drivers at the border crossing at Namangwa. I just drove from Nairobi to Arusha three weeks ago and crossed the border. It was not a bad drive (I am American and live in MN). I needed a negative covid test result and a yellow fever card to get into Tanzania and bought the visa for cash at the border.
Craig Beal - owner - Travel Beyond