To answer your questions:
1. I don't think work experience plays the biggest role in admissions (as long as you have shown interest in the accounting field). Although I do think Big4 experience is easier for adcoms to judge since most people are very familiar. I don't think it matters too much though and certainly wouldn't delay apps for a year to get some experience.
2. August is definitely ok. I took the GMAT in October and had no problems.
3. I think it's too early to give any definite school selections yet because you don't have a GMAT score. Also, you didn't identify what your research interests are. The schools you named are all fairly different so you definitely want to have a general idea of what you want to do (especially methodology).
Overall though, I think you're biggest concern (after nailing the GMAT
) should be to explain in your SOP why you want to do accounting research. I think adcoms will look at your profile and see HIstory undergrad, MBA, MS and JD and wonder where the research idea came from. You might have valid justifications that I just didn't pick up on, so this isn't meant to be a criticism.