How far is the drive from Miami to WDW?
4 hours minimum, depending on traffic.
Are cars easy to rent at Miami airport?
As easy as anywhere else, IME.
Would it be more expensive to drive back to Miami or fly out of MCO for the trip home? I haven't ever flown one way.
It depends on what days of the week that you fly, but the big advantage to buying round-trip is pretty much gone now; they have eliminated those one-way penalties for the most part. Also, time is money, and you will have almost a full extra park day if you fly into and out of MCO after visiting Miami. In any case I definitely would not drive BACK to Miami.
Can I check the car in at WDW and then re-rent for the drive back to Miami? Are rental companies within walking distance of maybe DTD? We don't use a car when at WDW.
Sure, and yes. There are also two that are on WDW property.
Any sites to see along the drive? Traffic? Best day to make the drive? The plan was to arrive at WDW on Sunday October 14.
I can't help you on the sightseeing, but IMO, your real best bet is to plan to fly from the Miami area to MCO. (The cheapest flights are not out of MIA, they are out of Fort Lauderdale or West Palm Beach.) However, to get the best price on that intrastate flight, it's best to do it mid-week; on Tues or Wed.
If you do drive it, know that most of the rental car agencies waive the drop-off charges between Florida cities, but not all of them do for Miami, so I'd double-check.