I have to disagree with you on this.
"Too often the rider gets to sit on a horse before he/she is shown how to sit properly. There is no question about what is the desirable and basic riding posture...It can also be seen...as demonstrated by an accomplished dressage rider..."
First, I agree with Littauer - riding is about balance IN MOTION. The motion of the moment, what you anticipate next & how your balance affects your horse's balance determines 'correct position'. Since dressage is about a certain sub-set of motion, the dressage 'position' is a subset of the correct positions.
"Whether one can copy the pose is another matter but for fundamental arena riding, the rider sits tall, erect, composed and relaxed"
Ummm...not really. If you could afford to relax in the saddle, much of the soreness would disappear. I don't get sore in a couch. Nor does motion alone cause soreness. I rarely became sore from the sitting part of flying in a jet fighter pulling 6-7 Gs or bouncing around in the turbulence that came when 100 mph winds hit the Scottish highlands. But then, I had a 5-point harness holding me to the seat.
When I took western lessons, and when my family members took western lessons, one of the first things they were told was that if it hurt, that meant something was wrong. And for a lesson spent sitting on a lesson horse doing circles using a western saddle and western style of riding, that was true. However, the horse IS using a lot of muscles, and as we learn to become a partner in the horse's motion, we need to use muscles too. And when you use muscles in a new activity, you can expect soreness. Doesn't matter if it is racquetball, pounding fence posts, jogging - if you use muscles more than you did before, you will experience muscle soreness. The large muscles tend to be used enough that they don't get sore because they are used to being used. It is the supporting muscles - the little muscles surrounding the big ones - that get sore.
Much of that is unavoidable. Our muscles get stronger in a limited range of motion. That is why you can't lift weights and automatically throw a baseball faster. When you bend your arm in a bicep curl, the muscle fibers you use at the beginning of the motion differ from those used in the middle or end of the motion. Riding a horse well, as an active member of the athletic team, requires many supporting muscles to do things not required by our normal activities.
"...Of course when one graduates on to jumping or cross country riding then priorities change because then the rider will be combating the effect of motion but constantly throughout a ride the rider must revert to the basic sitting position..."
No. Someone using a forward seat will balance differently than someone using a dressage seat or someone using a western seat because the fundamental desired motion will differ. At a walk, your position will be determined by what you need the horse to do or anticipate the horse doing. If I anticipate sudden acceleration in the near future, I will balance accordingly. If I anticipate a tight turn, my balance will differ. If I'm riding a spooky mare, my balance may anticipate a 3 foot sideways hop, or a spin and bolt, or a sudden stop - and guessing wrong may cost me dearly!
"If the novice learns at home to ride by trial and error or perhaps through the kind generosity of a friend then the probability is that he/she will not adopt the correct ‘position’."
I disagree. The less you pay attention to your 'position', the more likely you often are to get the correct one - by repetition. And that may vary as you progress.
For example, I started with very tight hips. 40 years of jogging daily can do that to a person. When I started, feet well forward was the only way my legs could relax. And when I started, feet well forward was right for me because a relaxed leg is better for riding than using muscle tension to force it to a position it cannot hold relaxed.
My feet still stick out too far to the side. Probably always will. With MY hips, toes out is a requirement. Forcing my toes forward WILL make me grip with my knee. Gripping with the knee is much more harmful to good riding than toes out.
This illustrates the problem with thinking about 'position' as a static objective, rather than something we do to balance in motion.
"An in-correct posture is a cause of back ache because the human frame has to learn to adapt to unbalanced stresses. It will be quite likely that human brain will in due course automatically compensate for the imbalances in order to keep the rider upright but the resultant fault will then become instinctive. Without a doubt once the newcomer decides that horse riding is a hobby for him/her, then a search should be made for a good - repeat - good and well recommended riding instructor who has access to a phlegmatic school master horse and a quiet, level rectangular arena. That is the required scenario for the rider begins to learn the alphabet of riding."
Your body is often smarter than you - or the riding instructor. Riding a "phlegmatic school master horse" in "a quiet, level rectangular arena" would have quickly convinced me I didn't want to ride horses. BORRIIINNNGGGG!!!!!!!
Nor would that riding be a very good preparation for actively riding a horse across rough terrain, or at speed, or for when the horse is apt to do unhelpful things like the famous Arabian Sideways Trot Thru Neighbor's Yard.
Position is linked to motion, because riding is about balance in motion. Instruction can be helpful, but only if the instructor understands what you will be doing and the horse you will be doing it on. And certainly for many of us, finding a truly good instructor isn't an option. Particularly for a new rider, because the new rider doesn't know how to evaluate an instructor. But knowing what I know now, I can't think of an instructor I'd recommend in the local area for more than about a dozen lessons for a beginner. After that, I'd recommend a year or so of riding before taking any more lessons.
"The novice should expect to ache after the first few lessons. The body is being subjected to new stresses for which the muscles may not be prepared. Develop the muscles both on horseback and in the gym under a Pilates instructor, then the aches will disappear. So much the better if the arena has a full length mirror at the end of the track then maybe the instructions coming from the instructor in the centre of the arena will be better understood. There is also the video camera."
I would as soon be emasculated with a spoon as ride in an arena with a full length mirror while an instructor tells me what to do. I'd MUCH rather pay attention to my horse, because riding is a partnership between rider and horse, and the horse knows how my balance affects her balance. After a dozen lessons or so, you've absorbed about as much as you reasonably can UNTIL your balance is better. You then need hours in motion, training your subconscious mind to control your body in response to your horse's motion. IMHO, an ideal training regime for most new riders would be a dozen lessons taken at 2-3 lessons/week, then repeated every 4-8 months.
"Personally I have come to the opinion that ideally before any new rider is allowed to sit in the saddle they must be taught in the classroom as to what is expected of them. Also I feel that before they ride they should take some physical exercise lessons under a Pilates instructor."
Yeah, THAT will get more men involved in our sport!
How did I learn to ride a motorcycle? By getting on one and riding. I didn't go to ground school, or take pilates at a gym, or do weight lifting with my legs. That isn't to say that I could not have learned anything, or that anyone would ever become a top motorcycle rider without any instruction - but the basics are not that hard. Get on a good horse and ride. If your hands suck, use a rope halter and try to keep slack in the reins.
Will you get sore? Of course. You are doing something new. Will you continue to get sore? Probably, unless you have the option of riding daily & for hours. And if you actively seek to ride in partnership with your horse, you may be MORE sore. A forward seat puts more stress on your back than a western seat because you accept that work from YOUR back so the HORSE won't have to do as much work with HIS back. Balancing on a running horse or one that makes sharp turns is more work than balancing on a walking horse.
Muscle fatigue is only bad when it comes from the wrong reason.
"Make the right thing easy and the wrong thing...well, ignore it mostly."