Answer question 1 + 2: Technically, both of you can submit the Express of Interest even though one of you will waste the spot (see below)
1/ If no one gets selected, good luck next timeIf
2/ If one you get selected, the other one could be a co-sponsor. The trick is between the two of you, you guys must decide who get to sponsor for his/her OWN parents first because one can only sponsor for his/her own parents (that's why the spouse can only be a co-sponsor). The rest does not matter.
Answer question 3: So long as you guys meet the income requirements for ALL people in your family for 3 years, you should be eligible to sponsor
Hope it helps