Quick answer to doubting the historical credibility of what the NT claims about Jesus:
These need some refining because there have been some arguments made which have by-passed some of the bigger concerns:
-Gospels are written by anonymous authors
That's at a general level quite true, the names were added later.
-Gospels don't claim to be eye witness accounts
Richard Bauckham does make a rather convincing case that the gospels contain within them testimonies from various eye-witnesses. The very specific and sporadic names within the various gospels lends to this view, the authors weren't just writing crap for no reason, it's altogether reasonable to assume that the names included within the texts were there to lend credence to the authors points.
-Gospels were penned, 40-70 years after Jesus died
We don't honestly know that actually, JAT Robinson makes a convincing case for an early NT. James Crossley, an atheist NT scholar, makes a very convincing case for Mark's Gospel to have been written within a decade of Jesus' death. At the very least we don't know when they were written and the arguments for a late date don't have the same air of certainty that they did last century.
-Gospels were penned in a language, Jesus or his supposed followers did not speak
We don't know this either. Jesus grew up only a few kilometres away from Sepphoris, a Greek city, we find burial remnants all around Palestine which suggest that Greek speaking may have been quite prominent, a small look at contemporary post-colonies suggests that the poor can learn the languages of their colonists, the Hellenistic presence in the lower Levant was centuries old at the time of Jesus, it's not unlikely that he knew Greek.
-Gospels have been shown to have additions or changes, which happened over hundreds of years
I wouldn't put a time frame on changes in the gospel traditions, they may be separated more by space than by time, but you're right, they do show evidence of religious evolution.
-Originals of the gospels are lost, we only have copies of copies of copies.
True again, we don't have the originals, they're lost. Textual critics think that they can be reasonably certain about what the originals may have looked like, I prefer the air of mystery in that we can't know.
-John is the gospel most questioned by historians, because of it's completely different writing style and how late it was written. It is the only gospel that claims Jesus was God and how could matthew, mark and luke have missed such an important point?
Again, we don't know when it was written, I don't immediately jump on the bandwagon of "it's mythical so it's late". From what I know of first century Palestine it wouldn't surprise me if mythical ideas went all the way back to Jesus' own mind.
Short list, as to why many NT historians, can't validate much of what Jesus did or said, with any level of historical credibility.
For the most part this is true. You just need to remember that there are still debates going on about much of what you've said though.