One big negative on your resume is that you lack serious proof based math courses like real analysis (I am surprised your UG math major did not require either real analysis or some similar proof based course). While this will not eliminate you from top 100 programs it will put you at a disadvantage and it will definitely lock you out of the top 25 and will probably lock you out of the top 50. Someone from a top tier UG could overcome the lack of real analysis with amazing grades and recs, but not someone from a non top tier school. The C- in math stats is a big negative, but I assume your Masters in Finance program will require you to take some econometrics classes and if you do well in those it should help overcome that C-.
The Masters in Finance will not really help your application. Those degrees (in the US) are very industry oriented and the coursework except for the econometrics classes have little relation to PhD coursework or research (and the econometrics classes are at a much lower level than what you would see in a PhD program). If anything you may have to explain in your SOP why you want to go for the PhD and not go to industry after going for a professionally oriented masters.
The research assistantship will help to show that you understand research and what a PhD is for. Ask the prof you are working with for schools with behavioral finance researchers. They are the most likely to be familiar with your professor and a rec from someone they know is worth a lot more.
GRE Q cutoff varies from school to school and while 85th percentile should get you past the cutoff at most top 100 programs, I would advise a retake to get over the 90th percentile threshold.
Your application as it currently stands is a very long shot at top 50 programs (you may have a very small chance at a school where the professor you are working for has connections), and even at top 100 programs you are only a so-so candidate. Taking real analysis will significantly help your application, but if you plan on applying this application season it will be hard to get that class in. You also need an econometrics class with a good grade to overcome the C- in math stats.