1. GRE vs GMAT - I'm not sure which is better. You may want to check the schools' requirements. Most schools accept both, while a few prefer one to the other. Regardless of which test you are taking, you should be in the top 5 percentile if you are aiming for the top schools.
2. I'm not in finance, so I'm not sure whether those MBA courses are quant enough for PhD. My impression is that MBA classes in general are not technical or demanding enough for PhD purposes (of course there are exceptions). Some of the foundation courses that I have heard for finance/economics include real analysis, computation methods, etc. I may be wrong, so you may want to check out other sources. E.g. contact the profs from your MBA courses - they should give you good insights.
3. It is never too late to switch. In fact, it is good to have some cross-disciplinary experience. For areas like strategy, I think some work experience is very useful.
4. Regarding your "chances", I suggest you check out gradjournal to see the profiles of applicants who have applied to the schools that you mentioned.