I have stayed at Fort Wilderness 7 different times and have never moved the car from our drive way once we got there. One of the main reasons we stay on Disney property is not having to drive anywhere. It may be faster sometimes to drive yourself to the parks but in my family's opinion it is worth it to take a little extra time and just ride the Disney transport. Relax - afterall you are on vacation

Plus you are paying for your Disney transport with your room cost so might as well take advantage of it.
I don't ever fly to WDW but I have read that there are some airport towncar services that include a 30 minute grocery stop on your way in to WDW. That could be a solution to your grocery problem. You could also take a taxi down to the CrossRoads (I think that is what it is called) across from Downtown Disney and go to their grocery store.
If you don't plan on eating a huge amount of food in your cabin - and if you and your kids are not extremely picky about brands - you could just shop at the Meadow and Settlement trading posts inside Fort Wilderness. The selection is not huge though and the prices aren't exactly cheap either - but it is still not highway robbery or anything. The prices are high on some things but in my opinion staples like milk and bread are reasonable. The first couple of times we stayed at Fort Wilderness we brought all of our food with us assuming that the prices at Disney would be extremely high. But now we have learned better and just bring our sodas and a few groceries - like canned pasta, snack foods and condiments - but buy our bread, meats, and dairy products at the trading posts. It just works out better for us doing that.
As for the off-site shopping - why in the world would you want to do that ?

Seriously though - I have never been to the Beltz outlet but I personally wouldn't waste my precious Disney time with a trip offsite to buy Disney merchandise. I understand it may be cheaper but it is possible to find clearance items inside the shops at the theme parks and in the resorts. Just look for the red and white price tags for bargins

I have been very succesful at getting very nice bargins inside "the world" several times by looking at the clearance items.
It looks like you have analyzed your rental car pros and cons just like I would have. I think it is really just a personal preference about driving. Some people swear to driving themselves around WDW and others - like me

wouldn't even consider it. Good luck in your decision
