The + for renting a stroller is that your older child could hop in if you wanted. The strollers are molded plastic, but if you aren't needing to nap in them--then I think they serve their purpose.
I would plan for two days at MK. I would also plan on doing a few hours, then take a break then a few more hours.
I think MK would be the most fun for your kiddos
Yes, you are allowed to take food into the MK. I don't know about Sea World.
Fantasy Land will be the main area that you will want to hit with small kids.
There is the Millinium Mall which is an upscale mall. We don't have those stores in my area so I enjoy shopping there. There are also a lot of outlets.