This is not the normal way the Israelites counted. If possible, try to translate in a way that preserves this use of the number seven. Alternate translation: "Seventy times seven years"
Here "carry out" is an idiom that means to accomplish. Alternate translation: "to accomplish the vision" (See: [[rc://en/ta/man/translate/figs-idiom]])
These words in this context mean the same thing. They ensure Daniel that Jeremiah's vision was indeed a prophecy. (See: [[rc://en/ta/man/translate/figs-doublet]])