Here is how I look at this issue and obviously enough people have already commented on it. Get what you can (or want) out of the Pali Canon and take it for what it’s worth. If it works for you or you get something out of it, that’s the point. If it doesn’t, you move on to something else if you choose to do so. The Canon isn’t even 100% there yet as translations are still ongoing, bear that in mind as well. The final one could very well be a “all of the characters and events portrayed in this story were fictitious and any resemblance to real people…” disclaimer.
I don’t think you have to become a Buddhist scholar with regards to the Pali Canon, just see if the core teachings work for you (i.e. Noble Eightfold Path, Four Noble Truths, etc.). At the end of the day, Buddhism isn’t about being able to impress people with how good you can recite a particular sutta/sutra or be able to find it in a book, it’s about being a decent human being (I’d argue the same thing for any religion frankly). For that, you don’t even need to read the Canon as far as I am concerned - most of the suttas are simply stories that detail how the various teachings were put into practice or advice the Buddha gave someone (or the rules for monks, etc.). I’m not saying that there isn’t some good stuff in them, because there is, but I think people get too hung up on trying to dig through them - keep it simple.
As for how accurate it is, I personally feel that it is not 100% accurate for a number of reasons. First off, the teachings were handed down via oral tradition for what? Something like 450 years or so after Gotama’s death before they were written down for the first time? It’s also known that the Buddha gave many teachings to many people that are not represented in writing (that we know of), so there is that to consider. I am sure that over a 400 plus year time span, things were ultimately written down differently or with with embellishments. However, something as simple and basic as the core teachings (i.e. Four Noble Truths) could have very easily been handed out accurately over thousands of years. In that regard, I think those are very much authentic due to their simplicity.
Second, regarding accuracy, most of the Pali Canon also starts off with “Thus I heard…” or something similar depending on the translation. This implies that “this is the teaching as I recollect it” and that means things could have been differently. So what you are reading in the Canon may not be 100% accurate, but it was the best thing available at the time and hence the “as best as I can recollect” line(s).
Third, Buddhist research is still ongoing today and there are always new archaeological finds being uncovered (goes for other religions as well obviously). This means that at any time, something new may show up on the scene that may completely re-write what we know of Buddhism today. Do we know for a fact that Bhikkhu/Bhikkhuni so and so was at a particular meeting for 100% absolute fact? No, because none of us was around back then. All we have is what was written down by some human being at one point and as I said above, that may very well be incorrect or inaccurate (I’m being realistic here). Again, you get what you want out of it and you take the good with the bad I suppose.
Question #2 seems to have been done to death already.
As for question #3, the teachings as they are represented in the Pali Canon are, supposedly, those directly offered by Siddharta Gotama. As such, I would say they stand alone as they are and what some other thinker made of them hundreds or thousands of years later is moot. This isn’t to say that other scholars or religious thinkers do not (did not) have valid things to add, but those authors are not the Buddha as he is understood in the Pali Canon. What’s more accurate? What I’m writing now as myself in the present or what someone 1000 years from now says I wrote or says I meant?
As for improving them, what needs to be improved? You have to realize (and I know this is a hot button topic for many traditions) that Buddhism, as it is represented in the Pali Canon, came from the historical Buddha as we know him and he lived in India. After his death, his teachings were picked up by others and transmitted around the world. In most cases, the Buddha’s teachings were incorporated into existing religions or cultural belief systems and you see the end result of that today in some of the traditions outside of Theravada. This isn’t to belittle or make light of other traditions, but you have to look at this historically and I would argue that it is all documented rather thoroughly if you do the research. However, the core teachings (i.e. Four Noble Truths, etc.) are the same across the traditions for the most part. The major differences are in ritualistic practice, different sutta/sutras, etc. Yes, I’m overly simplifying this again.
As for other Buddhas, if you go by what has been written, there have been countless Buddhas throughout history. Not all of them sought to teach others or have their teachings conveyed in writing or passed on, so who is to say which Buddha’s teachings are better than another? I’d argue that regardless of the incarnation of Buddha, the core teachings would be the same across the millennia. This is because the teachings are (supposedly) perfect and result in enlightenment, regardless of who is teaching it. I’d also go a step further and say that much of what Gotama taught was very much inspired by his Indian upbringing (i.e. Hinduism) and if you ever read something like the Mahabarata, you will see how similar Hindu teachings are to Buddhism (since the issue of Indian thought came up).
For me personally, I opt to follow the Theravada tradition because, as we understand it, it is the closest thing to the “original” teachings of the Buddha as taught in India. I do quite a bit of work for a monk in Thailand and have done my fair share of reading various Buddhist texts, including the Canon and other books written by scholars. I do not spend my days in front of my computer reading suttas or books about Buddhism (unless it’s work related), I go about my day trying to put the Buddha’s teachings into practice. The core teachings take about five minutes to read and you can find them on pretty much any site about Buddhism. Google “four noble truths” and “noble eightfold path” and they pop right up - you don’t need to go far to find them and those are the root of the Buddha’s teachings in my opinion, regardless of tradition. I do not think that immersing yourself in thousands of pages of material accomplishes anything other than providing you with being well read on the subject (not a bad thing, don’t get me wrong). If anything, I know it can confuse people due to the sheer amount available.
At the end of the day, if you don’t put any of it into practice, knowing suttas backwards and forwards or reading books by contemporary authors (nothing wrong with that either, don’t get me wrong) about it is moot if you do not put the core teachings into practice. I see too many people getting hung up on reading this book or that book by this PhD or that well known monk, all who proffer to know it better than someone else. The core teachings is where it’s at and they’re free. Buddhism is actually rather simple to follow and I honestly think people over complicate it rather badly.