It was 73967 that worked 1M16 solo. Other relatively minor (non-loco) issues contributed towards the 15-20 min delay, so it was a very respectable performance by a single 73/9 with Load 8 + full ETH, in freezing temperatures and in the middle of leaf-fall season.
No doubt there'll still be those who think 73/9s are weak, not up to it etc...
Not withstanding the above, it is supposed to be 2x (working) 73/9s on the Inverness portion. 73/9 solo + 73/9 DIT + Load 8 wouldn't be feasible. I'm not sure having three 73/9s on the front with the one DIT would work either.
As for the louder second loco - this may be because that loco is providing the ETH to the stock? Or just because some locos (of the same class) are noisier than others for whatever reason.