Earlier versions of TimelineJS required you to enter a date as January 1 of a given year to do this, but now you should just enter the full year, as explained above.
You can add B.C. dates by using negative years. For example, -100 would be 100 B.C. Be sure to change the formatting of the cell you want to modify to Plain Text — "Format > Number > Plain Text".
No, but it's pretty easy to copy rows into a Google Spreadsheet. TimelineJS uses Google's API to access the data from the Google Spreadsheet and turns it into native TimelineJS json.