Yes you can file prior to the first investment and we recommend you do. The states have the same 15 day filing requirement and you cannot file the states until the Form D has been filed with the SEC so any delay could cause a late state filing and possible late fees or other regulatory actions.
According to the SEC: the date of first sale is the date on which the first investor is irrevocably contractually committed to invest. If the due date falls on a Saturday, Sunday or holiday, it is moved to the next business day.
The following items can be considered securities: any percentage of ownership sold to another person or entity; stock shares, promissory notes, memberships (such as in an LLC or Partnership), real estate, debentures, options and warrants.