So, if I understand this correctly, I can use my existing receiver (Denon AVR-2112CI) in my 'MAIN ZONE', and add a Sonos Connect along side it for wireless streaming to all the other zones?
No, the Connect unit there is so you have access to all the Sonos content with your existing receiver. It can be played in sync with the rest of the house, and controlled from the same app.
Will the zones still be able to play off the audio from the 'main zone'? And then play independently while the main zone is playing something different?
Yes, you could feed your set-top box or other local source into the Connect's analog audio input, which makes that source available to any other Sonos device (in your case, the rest of the house). And yes, the Connect:Amp would operate completely independent.
And I would just need 1 Connect:Amp, which I could all place in a centralized place (probably my basement)? Or do I need a Connect:Amp for each zone? Will the sound quality be good and loud? I plan on putting a zone in my garage, want to make sure it will be loud enough in there and can be heard over people talking at parties.
You can start with one Connect:Amp and the impedance-matching volume controls as XJBaylor suggested. You can grow the system by adding additional units to allow more independent zones, or provide more power to one zone. It's for this flexibility reason why I wouldn't recommend starting out with a multi-channel amp. There are too many paths that can be taken that make it either redundant or unused... And it's money up front.
Instead of running bluetooth volume controls like before, I could run a standard knob-like control unit and avoid having to run all that cat5, since the Sonos acts as a wireless unit?
Run the cat5e regardless. You will want that cable at every keypad so you can upgrade those simple volume controls later.
Could each zone play their own audio, or all zones act as one? Speakers would still get ran to the controls, and then meet at this hub?
With one Connect:Amp serving all five zones, they all get the same audio. If you want more flexibility, you add more Connect:Amp units - to a maximum of five (such that each zone is totally independent).
And yes, home run all the speaker wire (and the cat5e from the keypads, too).
Jeff