Hi Irene,
Welcome and I hope you enjoy your stay here. Many people have replied already, but I feel that some of your questions have still gone unanswered, so allow me to attempt an answer as best I can.
Well, there are a number of considerations. The Buddha had many followers over a long time, and he frequently praised them and said they understand him perfectly.
There is nothing implausible about this—the Buddha was a skilled teacher who had many students. Why wouldn’t they understand him? The texts themselves bear this out: they are highly intelligent and complex, displaying at every level a mastery of textual structure.
Moreover, the evidence of the Buddhist tradition as a whole is that we have been pretty good at conveying texts in a fairly stable way for 2500 years, so it would seem a little odd if the first generation were the only ones who were incompetent.
Indeed. There were many monks and nuns who were not at the Council. But this does not, and cannot, mean that they had anything to do with the teachings that emerged half a millenium later, which we today call “Mahayana”. This is akin to saying that not all physicists of his time were convinced by Newton, therefore they must have been quantum physicists.
The canonical accounts of the monk who did not attend the council—whose name varies—say that he said the recitation at the Council was well done. Thus he did not contest the mainstream texts. One of the canonical Vinayas discusses what it says the points of which he differs are. One of them was whether it is allowable for monks to pick up fallen fruit from beneath a tree and bring it back to the monastery and have it offered by a lay person there. Well, I will leave that one for the experts!
The early texts are structured in a way that features massive redundancy. You can lose or distort a text; heck, you can lose or distort hundreds of texts, and still the main teachings would be clear.
If you are interested in such matters, check out The Authenticity of the Early Buddhst Texts by myself and Ven Brahmali. The basic thrust of the argument is that authenticity is not established by a magic bullet; there is no one decisive piece of evidence that proves anything. That’s not how history works. On the contrary, our picture of a period of ancient time is built up slowly and painstakingly, one bit of evidence, one suggestion, one hint at a time. The conclusion is broadly based on countless pieces of evidence. These are simply and reasonably explained by the obvious thesis: that the texts as we have them were the teachings of the Buddha as memorized and organized by his students.
This is why the conclusion is so robust for those who know; but it is also why it is easy to present one or other bit of evidence out of context and make it seem as if the whole thing is flimsy.
If you are interested to learn about the history of Buddhism from an informed Mahayana perspective, I would recommend the works of Master Yin Shun, one of the greatest Buddhist scholars of all time.
This is derived from late sectarian legends and has no basis in history. We have multiple early accounts of the First Council from many different traditions and none of them say anything like this.
Out of compassion to help sentient beings. There is no inconsistency; and Hesse and Toynbee are hardly experts on Buddhism!
The way karma works is that it produces, first and foremost, a state of existence, aka a life, which in this realm depends on a body. Cause and effect is not instantaneous and one-dimensional; on the contrary, it is deeply networked and ramified and it takes time to work out. The body has been produced and lives on, and an arahant looks after it just like anyone else would. Arahants aren’t monsters! It is just that one is not attached to it, so does not produce a new body in the future.
Perhaps it would clarify things to remember that the second noble truth refers to “craving for a new life”: that is what an arahant has extinguished.
This is subjective: it’s up to you. The teachings in the early texts themselves do, however, emphasize the importance of the Buddha’s own words as compared to those of disciples.
Later teachings, which include the very words I am writing now, are there to give perspective and enrich our understanding of the Dhamma. Personally I believe that every genuine Buddhist tradition retains the essential Dhamma teaching and includes many things that can enrich and support one’s own practice of the Dhamma. However, I also believe that the traditions have a lot of nonsense and silliness, not to mention outright corruption, and a measure of discernment and critical thought is essential.