I do not believe there is any way to know if Whitebox was an original buyer of part of the convertible deal, or bought them recently. Note that nwbo and Whitebox have dealings dating back to at least 2011:
https://www.nwbio.com/family-foundation-of-business-leader-invests-4-55-million-in-northwest-biotherapeutics-for-promising-clinical-trials-in-the-us-and-europe-2/
The escrowed interest is set aside in a separate account and drawn down as interest is paid. They initially put it on the balance sheet as a single line item "Interest in Escrow". Then they (properly) broke it into current and non-current lines labeled "restricted cash..." If you check the 10-K and 10-Q balance sheets over time, you can watch those lines drop as interest is paid. On the latest 10-Q, you can see what remains:
Restricted cash - interest payments held in escrow 397
Also note that $4.5m of the convertible was exchanged back in April and June of 2015. See notes in 10-Q filed 8/10/15 for details.
The term "bondholders affiliated" might mean one of Whitebox' limited partnerships.