I can answer a few of your questions as an alum.
First, Merchandise is a Global Role, so yes, you can pick up shifts in different resorts, DTD, and all four parks, plus the water parks.

I have not worked in Merchandise, but I know people who have, and they did that a lot.
The only complex that does not have a bus stop is Patterson, because they are close enough to share it with Chatham. It's less than a 10 minute walk, some people have said it only took them 5 minutes to get to The Chatham stop from Patterson.
I never tested it because I lived in The Commons. You are right about not all of the buses going to The Commons, but they DO have a Magic Kingdom bus. The Commons have buses to all 4 parks, the water parks, DTD, Wal Mart/Walgreen's/The Post Office / Publix and The Florida Mall. The only buses that do not go to the commons are The B bus, which goes to The Transportation and Ticket Center and Magic Kingdom, and some resort buses.
Magic Kingdom has two buses. Both the A Bus and the B bus go to West Clock (Backstage at MK). The A Bus also goes to Fort Wilderness and The Contemporary, the B Bus also goes to The Grand, the Poly and TTC. Both buses have access to get to MK as a guest in the front of the park.
I normally took the A bus since the B bus did not go to my complex (the commons), i would get off at the Contemporary and walk the sidewalk to MK. it took me around 10 minutes to get to MK's front gates that way. You could also ride the monorail from inside the contemporary, but I never got around to doing that.
Once, when I took a networking class at Vista Way, I caught the B bus to Magic Kingdom. I got off of it at The Poly and took the monorail from there to the park.
If you have any other questions feel free to message me.
Also, congrats to everyone on their acceptances. I found out on 10/2 that I got accepted for my second DCP!!! Can't wait to go back. Going to be a Photopass Photographer from February 2 to August 7, 2015. I cannot wait!!!