In the 1892 RLDS edition, the and at the beginning of this sentence was accidentally shifted to later in the sentence, creating a dittography (“thus ended the seventy and and seventh year”. This error was not noticed because the end of the line came between the two and ’s. There doesn’t seem to have been any intent here to consciously delete the and at the beginning of the sentence.
In the original text, there are 50 instances of “and thus ended something”. But there are no instances in the original text where a clause actually begins with “thus ended something”. There is one case where there is a preceding “it came to pass that”, but that too has an initial and: “and it came to pass that thus ended this year” (Helaman 11:32). There is one case, in Alma 16:21, where Joseph Smith removed the and in his editing for the 1837 edition; in that case, the and also served as a Hebraistic separator between a preceding present participial after- clause and its following main clause (for discussion, see under that passage). The point here is that no clause in the text ever begins with “thus ended something”; there is either an immediately preceding and or the Hebraistic “and it came to pass that”.
Summary: Maintain the occurrence of and before “thus ended the seventy and seventh year” in Helaman 11:21, the earliest extant reading; for all other occurrences in the original text of the expression “thus ended something”, the clause begins with an and.