I'm a horsewoman who has shown jumped and ridden horses, and I'm no expert on everything, but I do have experience and a point of view that is different than yours.
I'm probably on the soft side on the way I treat my own horses. I don't know anyone who loves or pampers their horses more than I do. Even the bad pony who has no purpose in life here gets the best of care.
And I admit that people in the horse business are as good, or as bad, as people can be in any other facet of humanity. There are people who abuse horses. There are people who are just dumb about them and their horses suffer for it. BUT by and large horse people are sensible, sensitive to the needs of the animal, and interested and prideful of their level of professionalism and animal husbandry. By and large, I respect the people who have horses and think most people will try to do right by them... The ones with a backyard horse, all the way up to the ones who have reached the top of horse sport. So now let me answer your points one by one:
The manner in which race horse have been treated over the last century is despicable. All racing breeds - Thoroughbreds, Standardbreds, Arabians, and Quarter Horses - are subject to inhumane treatment. The abuse ranges from neglect and simple lack of education on the part of the trainers, to violence against the horses, as well as dangerous - sometimes deadly - drugs.
I would argue that despite the bad seeds that act out of meanness and selfishness, the state of horse care in this country. The state of the horse has NEVER been better. Horses are expensive luxuries, and by and large horses, even race horses, live longer and healthier now than they EVER did in the past. There are LOTS of people watching horse sport. Abuse is NOT respected by their peers, it's criminal, and abusers don't make it.
Horses - especially Thoroughbreds - are started under saddle around the age of two; this can be up to five years before a horse's skeleton is fully developed. (The human equivalent to this would be for an eight-year-old child to run around with a 50 pound sack of coal.)
I will agree that it would be better if they were a year older to start running hard, but at two, a TB is not equivalent to an 8 year old, it would be more accurate to compare them to a 17 year old. They are nearly full size, fully sexually mature, and are typically started under saddle at two to three in other disciplines as well as racing. Most pleasure horses are started slower and not worked as hard until about 4, that's true.
Thoroughbred, Quarter Horses and Arabians who race under saddle are not taught how to listen to their rider, or do anything but gallop at top speed, WHICH CAN CAUSE SEVERE LEG DAMAGE.
Not really true. While they aren't finely trained to do a lot that is complicated, they are barely broke off the track, they don't actually run at top speed all the time, they go for several types of workouts in a given week, some walks, some trotting, some with a pony horse, and lots of easy cantering. Not just hard galloping.
The constant pounding of the legs on a hard track can often cause tendons to bow. If left untreated, a damaged tendon can cause chronic pain and permanent unsoundness. The two main ways that a lame racehorse is dealt with are pin firing and nerving, both of which are extremely cruel to the horse.
Yes, racing can cause bowed tendons. Bowed tendons are painful while accute, but often heal nicely and without further problems. My 24 year old Arab has an old one he had when I got him at 10. No problems.
Pin firing and blistering are old methods for treating all kinds of lower leg ailments. Less in favor now than it used to be, because it does leave scars. You are right that the point is to create a soft tissue injury that will increase blood flow to the area because the lower leg otherwise has a poor blood supply. It sounds scary, but it also saved horses I think. I'm pinging vetvetdoug, because the part about the acid is new to me, particularly with the term 'nerving'. Nerving as I know it, is where the nerves, usually to the foot, are surgically cut, as a last ditch effort to save horses with chronic, painful foot problems such as founder and Navicular Syndrome. It does numb the foot. The alternative is severe pain or death. Anyway, I don't know for sure about the 'acid' you post of, so I'm pinging a vet, and hoping he can answer.
For many owners, the race horse is a financial investment, not a living animal (the way many people see their trucks or cars). Many owners do not have the money to buy another horse if theirs is not fast enough, so their horse is forced to run even if he is unwilling or in pain.
Horses are an investment indeed, and some take better care than others, certainly. I would propose to you though, that in this day and age, with the feed, medicine, expertise and animal rights 'watchers' everywhere, while there are still bad things that happen, over all, life has NEVER been better for the horse. It is unfair, and inaccurate to paint horsemen as characteristically abusive.