The problem is that vast sections of Mark that appear in Matthew and Luke aren't only just reporting the same thing, they are word for word copies of Mark, then added on to with additional narrative. That suggests that Luke and Matthew are plagiarized and then expanded upon subsequently. This is known as the synoptic problem, and is well known to biblical scholars.
Likewise, it can be demonstrated that the author of Luke lifted parts of Matthew, and other secular writings, like the historical writings of Flavius Josephus, who was a prominent Roman Historian.
As for John, you are correct in that it is much more independent, however it's not completely independent as you suggest. Some of Mark and the other gospels to a lesser extent also appear in John. Interestingly enough, the Gospel of John was deemed heretical by some prominent early church fathers, however it wound up garnering enough support to make it into the canon.
Which short ending though? There are a number of different endings that we know of in different works. There's actually an expanded long ending as well in another manuscript. The resurrection narrative however (Mark 9-20) is universally regarded as a later addition
That's complete nonsense. The above described ending to Mark is one such example. We also have writings of the very early church fathers like Papias who quoted the gospels of Mark and Luke, however his quotations do not appear in the gospel of Mark or Luke as we know them.
There are a multitude of interpolations, edits, additions and deletions to the biblical manuscripts.
Ari in Greek
https://en.wikipedia.org/wiki/Ari_(name)#Greek
Mathetes in Greek
https://bible.org/seriespage/2-understanding-meaning-term-disciple
and "ea" is a common ending of biblical towns or lands, for example Judea means "Land of the Jews".
Not to mention that the location of Arimathea is intentionally left vague in the bible, only being described as a small Jewish town. As all four gospels were originally written in Greek, this is the Greek translation of the phrase.
It's not totally unexpected though, there are other characters in the bible with fairly dubious names that were almost certainly added later. For example, "Lazarus" means "God has helped him".