With 20 knots op the chuff, there will be exhaust fumes blown back into the aircon intakes (belly of the 757)
Incorrect, any smell entering the ram air ducting is exhausted from the ram air exhaust and never enters the cabin. However any smell entering the engine intake may indeed make it's way into the cabin, fully understand what was being eluded to though.
Hedgedweller IMHO should be asking why the engine failed, a report will hopefully be in the public domain eventually.