Can any spell be played as a face-down 2/2 creature for 3 colourless mana?
No, you cannot generally cast any card as a face down spell. Rule 707.1 is saying that there are some cards with abilities that specifically allow you to cast face down spells or otherwise get face down cards on the battlefield. The Morph ability itself is what allows you to cast the card as a face down spell, and that only applies to the card it's printed on, not any other card. The keyword action Manifest on cards like Ethereal Ambush puts cards on the battlefield face down and the ability on the card Ixidron turns cards on the battlefield face down.

Rule 707.7 simply means that the rules themselves do not provide any way to turn face down spells face up while they are on the stack. There are also currently no cards that do that, but the wording of that rule leaves that possibility open for the future.

Rule 707.11 is just describing how to handle "As [this permanent] is turned face up..." abilities. Many morph creatures have those kinds of abilities, but many don't have them. Cards without Morph or Megamorph don't have abilities like that.