BEFORE ANYONE COMMENTS. This is my opinion. It may clash with yours and if it does do not post hateful comments telling me how wrong I am. It is only an opinion and I'm entitled to have one.
Ok. My take on Fluffy/Kiara theory.
Simba and Nala had twins. A boy and a girl. The girl was the younger Kiara and the elder was a boy named Tanabi. It was Tanabi who was presented at the end of TLK, either the day he was born or the day after, since he was the elder and therefore made the heir.
However, as Tanabi and Kiara grew, Tanabi began to show signs of illness. He was prone to epileptic (spelling?) fits and never made noise unlike his perfectly healthy sister.
Simba and Nala were forced to make a tough decision. They could either stick with Tanabi as heir and risk that he may not make it to adulthood or proclaim Kiara, their healthy cub, as their heir instead. With heavy hearts they made Kiara the heir instead of Tanabi.
So the animals were called to Priderock again, Kiara an older cub now, and she was anointed as future heir at the beginning of SP.
Kiara and Fluffy belong to Disney
Fluffy's design here belongs to me