|Mathwei ap Niall|
1. As you explicitly pointed to Natural Spell Combat (Bite) as something that would never work without your ruling, pointing out that it does not work without Natural Spell Combat is simply a statement of fact, regardless of the status of rulings regarding hands. You do not Bite with your hands.
2. "Functioning as a hand" and "associated with a hand" are not the same thing. A prehensile tail can also function as a hand with the right feats, but it is not legal for use with Spell Combat.
3. Excellent. Now, if that rule regarding unarmed strikes is true for everybody, explain why it got specific mention as a special ability of the monk? Given that the rule is in the combat section explaining what you can do with an unarmed strike, rather than what it is, it doesn't really help your case... but then we still have the Monk issue.
Really, this is all rules lawyering of the worst sort. An attack associated with a hand doesn't actually use that hand? Really?
You are adding conditions to the ability that don't exist. There is no requirement for it to "Function as a hand" it simply has to be associated with a hand. The Developers have stated this over and over Claws work with Spellcombat, Slams work with Spellcombat and all Unarmed strikes work with spellcombat. Why it works is unimportant, they say it works so it works. Period.
Monks get a special mention because they can do everything everyone else can and more. Normal characters can't use Knees and Elbows with UAS Monks can. Normal characters provoke when making UAS, Monks don't and Normal characters do 1/2 damage with their offhand attacks, monks don't. THAT'S why monks get a specific writeup, they simply do it better.
If you want to play a game where the rules of the world match ours then go play life. DnD/Pathfinder is a simulationist game with different rules and a more flexible meaning of the term WHY.