Congrats on having a 21 year old companion
and welcome to the board.
In the blood tests did they check the T4 aka Tetraiodothyronine to test for HyperThyroidism? This test is sometimes missed because for some labs it is an added test.
Her kidney & liver enzymes were normal? Anything out of normal range on the blood tests?
Was there a urinalysis done to test her urine?
What brand/flavour food are you feeding? Wet or dry? and did the diarreah start before or after switching to this new food?
Does the eating of other things happen right after your cat eats? Is her stomach tender/sensitive/painful to the touch?
How is her water consumption? Peeing normally?
Try elevating her food & water dishes so they are about 2-4 inches off the ground by placing a phone book or something under them but ensure they won't slide off. If your girl has any nausea while bending down to eat/drink the elevated dishes will help keep the stomach acid down in her stomach, and even if there is no nausea it will make it a bit easier on her neck to reach her dish.