What % of a monthly payment under IBR is applied to interest. How is DL (or any lender) really splitting this under IBR? At the end of the year, the lender will need to produce a 1098-E for paid interest.  How is this determined under IBR?

Based on the Simple Daily Interest Formula (http://studentaid.ed.gov/PORTALSWebApp/students/english/MonthlyInterestCalculation.jsp), here is the example DL uses:

32 (days) x $9,500.00 (PBO) x .00022587 (interest rate factor) = $68.66 of you payment applied to interest.

$160.00 (payment) - $68.66 (interest) = $91.34 applied to your principal loan balance

This is fine, but what if the PBO is $180,000, and under IBR, the monthly payment is $547.  What % (or amount) of $547 is applied to interest?  Even with negative amortization, is any amount of the $547 payment applied to interest?

IBR does allow for negative amortization and for borrowers to pay less than the interest that is accruing.  The unpaid interest does not capitalize unless or until the borrower either 1) chooses to leave IBR and switch to another repayment plan, or 2) no longer has a partial financial hardship but remains in IBR.  Upon either of those triggering events, the interest is capitalized, but not until then.