Just read Denzel's story through on here. I am glad you are feeling better about things now and good on you for not pushing him into being the perfect pet (some do)
Living in Bulgaria we see thousands of what you call 'chain' dogs, pitiful scraps who are usually tied incredibly short so unable to do the job they are supposed to be there for (guard), and vulnerable to attack by other dogs and jackals. As you mention they are rarely touched even though the law states they should have all the things a UK dog has, including shelter, appropriate food and all the vaccinations needed here. They are lucky if they have an old piece of wood to shelter under and are fed bread and milk or whey with the odd bone if they are lucky. They are tough, often small as Bulgarians are frightened of dogs, especially big ones, and have the most awful mental and physical ailments, as you have found. (the strange way he held himself could have been because he was constantly fighting a very tight chain from a very tight collar. The same happens to foals as they have to go everywhere with Mum, who is back to work as soon as te afterbirth passes, foal tied to mum)
But by and large the population are not cruel, it's just the way things have always been done and in the villages they are very insulated against education, extremely poor, very private and worried that thieves and predators are everywhere, hence most houses are surrounded by high walls and metal gates. Village houses have around half an acre with buildings, stock live in sheds except when the sheep/cows/goats are taken to graze by the village shepherd. It has always been this way and will take a long time to change. The reverence our neighbours show their and our animals and their veg is something to behold. They adore them and are not there at slaughter which is done at home. They love their nine month old bitch too, but have been frightened of her from six week old fluff ball, horrified when I picked her up for a cuddle. Her story so far is not for here, maybe I will tell it later. She is a chain dog who lives under a trailer, they talk to her all the time but are scared to touch. They are not cruel, just uneducated and only do what their ancesters do. So please don't judge them by our standars.
Hopefully change will come and Denzel's peers will have a better life in the future. Maybe the above will go some way to explaining why there are so many poor dogs like him. I do admire you taking him on, but at the same time he must have been highly traumatised from being freed from the only security and life he ever had, handled, then driven in a tiny (to him) box to more strangers in a strange country and a house . Bless him and you.