Hi Mina,
First off I'm not sufficiently qualified to address your questions so take what I say as a friendly opinion.
Given what I have seen seen around this forum, these schools are competitive. There are very few make or break applications.
Since you have attempted the GMAT four times already, the univeristies you choose to report your score to will see all your scores( including all your IR scores). I don't think there is flexiblilty is influence the reporting outcome. Having said that I don't think you'll be harmed if you decide to attempt the test again if you wanted. But it is also quite unnecessary.
Like many others, I will also advise you too concentrate on your essays and how you want to present yourself to the admissions committees. Having devoted so much resources on the GMAT side, you would do well to spend equal, if not more, time on getting your application together.
Lastly, put your best foot forward because the top 20 is appropriately within reach for given your background.