How is short interest paid out, & is it possible to estimate the amount of short interest being paid?

You're confusing two market metrics. As Joe stated in his answer, the Short % of Float is the number of shares shorted divided by the Float. However, no one here has yet to explain how they arrive at the calculation offered on web sites like Yahoo comes from.

When shares are shorted, the borrower pays a borrow rate to the lender. The borrow rate fluctuates and the amount due daily per share is the borrow rate times the closing share price divided by 365. And yes, you pay it for the weekend as well.

I can't speak for all brokers but at my primary broker, it accrues daily and the total is deducted from my account at the end of the month. It is not compounded.

The aggregate amount of interest owed on short stock each day would be the the above calculation times the number of short shares.

The borrow rate for the past 5 days at my broker (not including today) was:

 2/02 16.90 %
 2/01 18.81 %
 1/29 31.73 %
 1/28 37.12 %
 1/27 50.83 %

It is not possible to estimate when and at what price stock was shorted. Only the stock exchanges know that info.