Let me see if I can help. (That is assuming I understand it to begin with!)
Starting with your first question there are a few different pay catagories. Each has it's own advantages and disadvantages. We're cruising on the Ecstasy in November. I'll use that as the example for pricing. (All prices were pulled from the Carnival Website)
The Fun Saver price is normally referred to as a Guarantee Room. An Ocean View room (OV) will run $409. Your room can be on any deck of the ship, you won't know until it's assigned for you as you near your cruise sail date. You might receive an awesome upgrade.
The Past Guest rate is just that. A discounted rate for folks that are loyal to Carnival. An OV room on the Riviera will cost $389 and you get a two catagory upgrade for free!
The Early Saver is a great deal if you know for a fact you will be on that specific cruise. There is a non refundable deposit required (normally around $200) but, your price for an OV on the Riviera deck is only $309.
To answer your other question....I like to compare apples to apples. So, if I pay the Past Guest of $389 and end up on the Upper Deck with the upgrade (porthole window), I would check on the Early Saver price for the Upper Deck. (For a porthole room on Upper Deck it is $309.) Your Fun Saver rate is $409 but as I said earlier, you might be on the top deck or the bottom. (Any deck is great in my opinion).
The hard part is to decide what is best for you and your family, the easy part is picking a cruise as your best vacation ever! Please let me know if you have any more questions.
I hope this has helped and if anyone else out there wants to chime in, please do so. I just know how confusing the pricing can be.
Enjoy your cruise,