You are clearly a very strong student. I think that you will do well wherever you end up. You are competitive for Harvard.
Usually we can only say either “no chance” (does not apply in this case) or “your chances are similar to the acceptance rate”. However…
This is the unknown. If you have done a very good job for this particular professor, then this could help you quite a bit. “…with a small published paper” could help you also. The details will matter in both cases.
If you are confident that you want to attend Harvard, and that you can afford it, then to me it looks like it is definitely worth the REA application. I do think that you should make sure to also apply to safeties. Depending upon how much you like your safeties you might want to also apply to match schools.
Congratulations on your successes so far. Keep up the good work!