People often confuse trademark, patent and copyright and what they cover and can protect. An "idea" might be something useful or functional, which is covered by patents. A trademark is directed to words, phrases and logos that identify a product or service, such as McDonalds or Nike.See question
The short answer is maybe. There are rights of publicity, the scope and term of which vary state by state, and potentially trademarks to be concerned with. Each individual would have to be analyzed as to how long they have been dead and if they have any right by publicity or trademark. An intellectual property/trademark attorney can help you with this.See question
The above answers are instructive. It might be possible that the illustrations and description support the addition of a sentence or two, but perhaps not. A patent attorney would need to review to give an opinion as to whether the proposed additions would constitute new matter and be rejected. As mentioned above, a CIP would be a good alternative approach.See question
Typically, you would need to hire an attorney to look into the status of patents. However, this took but a minute on my part and I can tell you that the patent in question was issued in 1920. As patents have a potential term of 17 to 20 yrs, it expired quite some time ago and is in the public domain.See question
This is a complicated fact pattern, the details of which you should discuss in private with a patent attorney. There are issues of public disclosure, timing and who contributed what which can impact who might be able to patent what, if at all, etc. that need to be ironed out, and best not on a public forum as this.See question
There is a good chance that there is infringement, although you should have a trademark attorney look into various issues such as first to use, similar marks, etc to confirm before sending out a cease and desist letter or taking other action.See question
It does appear that there are facts which could support an Opposition in which you could be successful in prevailing. If you do prevail, and if your competitor has not actually used the mark in the U.S., your intent to use application could have priority. You should discuss the details with a trademark attorney.See question
Your approach depends on how committed you are to your mark and how much money you might want to spend. The best advice would be to consult with a trademark attorney, who can do some searching and give you an opinion. The advice might be to withdraw your application or it might be to engage their attorneys.
Regardless, if you don't withdraw the application you run the risk of having your trademark application opposed, if it is allowed, or being sued for trademark infringement, either of which could be expensive.
If money is a concern and you can select a different trademark, you might want to withdraw/expressly abandon the application.See question
You can obtain protection for a trademark that is not limited to font, size, color or the like. There is a question of whether your use is as a trademark or ornamentation, as mentioned above. You should consult an attorney to see if your use constitutes a trademark and also consider a search to see if the phrase is available to be registered.See question
Interesting idea. However, as mentioned above, you cannot copyright ideas. If you create a website or other materials, you could copyright those, but it only protects the material itself, not the actual style of riding.
Trademark could be used to associate a name or logo with your teaching or riding style, but just prevent others from using that name or logo with similar services.
Patents protect ideas and inventions, but there are limitations as to what can be patented and a teaching method or style of riding might not qualify for patent protection.See question