Mrs Ojo,
You sound like me with all the analysis, LOL
. I vote for DOOR #3 -- bringing your umbrella stroller from home for dd2 to be used for activities OTHER than going to the parks (like around the resort, DTD, etc). I'm assuming that DS5 is not opposed to walking on an "average" outing.
But, when going to the parks, I'd prefer to rent the double stroller. I wouldn't want to have two strollers -- NO WAY!!! I think that would be a mistake, unless you have more than two adults. What if you or DH wanted to run for the bathroom, or a fastpass and meet you somewhere. You or He would have to take one of the strollers. It would limit your options, IMHO. Plus, if your kids are like mine, they will prefer to be together.
The only downside of renting in the parks is that you will have to manage your DD2 to/from the buslines. It's the price you'd have to pay. I think it would be worth it. Good luck to you !!
Jackie