Hi! Thanks for sharing your story. You have a really difficult life-choice decision to take.
I would say it depends on whether you would be able to increase your GMAT in less than 6 months and improve you profile (you may consider using consulting services).
There are some other questions I would like to ask you
1) How many times did you take the gmat and how much did you prepare for the test? Do you know your weaknesses?
2) Do you believe you are capable of getting 720+ in less than 6 months? (I assume you will apply in R1 or R2).
3) Do you think you will be very competitive candidate for top 10 if you get a 720+ on the gmat?
4) what is your age? The older you are the more difficult it would be to get into IBD. So staying for one more year may be an issue.
5) do you want to work only for very top/prestigious IBanks? (bulge brackets)
6) Do you think that after graduating from let say Cornell and the GMAT 720+ you will be competitve applicant to bulge bracket banks?
7) how bad is current economic situation in your country? woub you be able to stay there for a year?
worst case scenario is that you will retake the test, but not attain a 700+ score and reapply again to GT next year however I would say that after rejecting their offer once they may not accept you next year. Then you will apply to other even lower ranked schools from which you would not have a shot to get into IB.
Ultimately it depends on how much you are risk averse person.