No, having more borrowers on an application doesn't get you a better interest rate. In some instances, having more borrowers might get you a worse interest rate depending upon the credit scores of the borrowers.
It depends upon the number and when they are pulled. For hard credit inquires, like inquires in a short period of time will be considered one inquiry against your credit score.
It depends on the type of mortgage. For most mortgages, lenders are required to count a payment for student loans regardless of the status of repayment.
