The situation on the motorway sounds awful and apart from the casual comment from the owner that the horse travelled better facing backwards there were really no warning signs especially given that the OP mentions that the owner had a trailer themselves. the fact that the pony broke through to the living of a horsebox doesnt really make it sound like the pony travelled ok on a box either
The turning out could have been handled better maybe but hindsight is a wonderful thing
I too am baffled by the criticism towards the OP. By the sounds of it the OP was VERY lucky that they managed to make it home with the trailer and with horse and people unharmed.
I also have a mixed herd and think this is the normal way to keep horses. I would probably split the herd in two and introduce new horse to a smaller group first, but I have also put newbies in with the whole herd in the past. Plenty of biting and squealing in to be expected. A horse penetrating another is NOT normal, and this action alone would cause me to return the horse immediately
I would have been terrified if that had happened to me on the motorway! Saying 'good to travel' is a bit misleading, why put it in the ad? Just leave it out and explain he needs to travel backwards but otherwise fine. Most people are prepared to make some compromises for a loan if it is otherwise good.
Oh and yes I would say something. I can't understand why people bother loaning a horse out and lying, it'll only get sent back which is surely a load of hassle for everyone.