I used chrome. The process went smoothly for me.
As for additional items, I chose the following.
1. Dopey logo pin bundle. I get a pin from every race and prefer the logo pins.
2. AP pin. This may be the only time in my life that I'm a Disney World passholder so why not commemorate it. Also this race played a huge part in me deciding to get the AP.
3. Race retreat. If being able to actually sit down for at least part of the mornings of the half and full helps preserve my legs to finish the full it will be worth it.
4. Race weekend jacket. It's my first ever marathon. If I don't like it, I can always return it.
I like the banner artwork that
@SarahDisney posted earlier.
I passed on the shoe ornament. Assuming I finish, I plan on buying one of the ornaments in the Magic Kingdom Christmas shop that comes with customization. I also discovered that if you like the free Disney celebration buttons, that same shop has all sorts of race related I'm celebrating buttons.
For corral placement they wanted proof of time for under 5:30 and if you didn't have that, it appeared that they wanted proof that you could finish in the time you entered instead of placing the best corral without proof. But maybe that's just my interpretation.