Hello Reddoodle_7, welcome to the forum and I am glad that you decided to post because you have really done your homework and your questions are valid ones.
I don't know that I have sufficient answers for you, but I'll try...
The first thing I'd like to address is that you are right on with what you talk about finding a good breeder, the testing, the specific health warranty and not breeding until the dog is mature.
Now, finding the correct timing for "maturity" is a good question...as a general rule, it used to be (and many still believe it to be) that dogs should not be bred until at least 2 years of age. I think that there are two reasons for this time frame. 1) The dog is mature enough to have and care for puppies by this time and 2) testing can be completed.
About maturity, when a breeder waits until the second heat, they still take into consideration (or at least they SHOULD take into consideration) the actual age and maturity level of the dog. Some dogs have heat cycles every 6 months and some have them every 8 months or longer.
I personally believe that if a breeder has raised the female, the breeder has a good indication of what this particular dog is capable of, by way of maturity. For instance, I bred Lexie on her second heat, and I discussed it very completely with my vet, I made sure that she was healthy and capable of going through a normal pregnancy...more importantly, that she was mature enough to care for her puppies and to nurse and nurture them. Often if a female is not mature, she will abandon the pups or stop nursing/cleaning them.
The decision must be a personal one made by the breeder and if you feel that the breeder has been irresponsible, then you should buy elsewhere.
In my case, Lexie was ready to become a mother...my vet agreed with me, and in fact she was an excellent mother.
About the second part, I believe (others will disagree with me) that the 2 year mark was set before PennHip exams made it possible to test hips of dogs younger than 2 years of age. You are correct that OFA can only do preliminary hip testing before 2 years...but PennHip tests are done at 4 months of age. Therefore, a PennHip exam will remove a female from the breeding poole at an early age. (Neither PennHip nor OFA are foolproof...but it is the best we have.)
So, 2 years was the breeding limit set because responsible breeders could not have the hip results until that age. Now, it is different and hips can be tested earlier, so the next important issue is maturity.
In answer to your next question, which can be very complicated and scientific, I will simply give you a web page that I beleive explains why breeding to unrelated breeds is preferable (healthwise) to linebreeding (which is often done in purebred dogs):
http://www.akitarescue.com/Canine%20Div ... roject.htm
I hope that this helps a bit.
The thing you will find is that people (breeders, owners, experts, scientists, vets...) will disagree with one another on most topics...from the health of mixed breeds to the type of dog food to feed...but you have to do the research, as you are, and find the answers that work for you...then find a breeder who agrees with your opinions/beliefs.
The most important factors, once you find a like-minded breeder, are to find someone who will stand behind you and your puppy for the life of the dog, and one who will offer a good warranty and who has the integrity to follow up in a way that is morally responsible.
All of this is the same no matter how long the breeder has been in business...I personally have only bred one litter, with another on the way...my inexperience makes me very cautious and I want to learn everything I can, but it doesn't mean that I am not a very good and capable breeder...I love my dogs very much and want to protect them.
I have known of long-time breeders who know less than I do about the subject...so it isn't always about the length of time a breeder has been in business, but the dedication to learning, their interaction with other breeders and their ability to love and care for their animals that are most important, in my opinion.