Are you still looking for a solution to this? If so, are you scheduling historical activities, or are you simply trying to move the Project Start Date back to some point before your earliest scheduled activities (e.g. NTP Milestone)?
In either case, it seems you may not be clear on how Project Start and Data Date work (and maybe progress update in general). The Project Start date identifies the start (or earliest) point from which you will schedule project activities, and P6 will not let you schedule activities prior to this date. The data date does not "constrain" the project start. You can move the project start back any time. The Data Date (aka Status Date), is the earliest date from which remaining work will be scheduled. The Data Date can be thought of as the line between the past and the future (progress and forecast) in the schedule. The Data Date is moved forward at each update in conjunction with applying progress to activities (i.e. Actual Starts and Finishes, % complete, etc.). Any activities not marked "started" and/or "finished" will automatically reschedule to the next earliest dates ON or AFTER the Data Date. There are other things to understand as well, but I suspect the main problem is that you are creating historical activities but not applying actual start and finish dates to before moving your Data Date.
Let us know what you've figured out and what else we might help clarify for you.