Hi Rachael. From what you wrote, there are only two options for the answer you want - {When did Inventory Land?} or {Updated Estimated Delivery Date}. So, you could use…
IF(AND({Inventory Arrived?}='Inventory Arrived', {When did Inventory Land?}),
{When did Inventory Land?},
{Updated Estimated Delivery Date})
Let us know if works the way you want. Good luck.