Dear Elsie,
I sincerely doubt the majority of errors were purposeful. Medieval scribes often could not read well enough to understand what they were being asked to copy in scriptoriums. Over the centuries the forms of letters changed, too, complicating the task of the scribes.
As for deliberate changes or additions...it isn’t my place to tell you as biblical rsearch continues apace. The discoveries of documents, some previously known and others unknown, expands the knowledge biblical scholars have concerning the first couple centuries after Jesus’s death and particularly the few decades prior to his preaching in Jerusalem. It is best you make this research on your own.
I find this hard to believe!