After dinner you won't be looking for an AK bus...you'll be taking a bus to the TTC or DTD. Those buses run late.
But I would grab a cab back - much easier.
I always wear "resort casual" inside the parks. Going on 40, short shorts and tank tops just don't do it for me - and I've never looked good in a tshirt collar. I guess it depends on what is meant by park attire. My mother looks fabulous all the time - I don't know if she owns anything that isn't at least "resort casual" - and would only need to change for V&As (she is also one of those women who don't sweat - at least not without intending to). My sister in law on the other hand doesn't own anything that is "resort casual" - she is strictly a tshirt with writing across the front of it kind of girl - collared shirts are for yuppies. It also depends on when you travel. Even dressed well, a shower before dinner when its 98 degrees in July might be a good idea if you are upscaling from Cosmic Rays, but if you are traveling in November, it probably won't be an issue.