When I marry her I will divorce her; it is not a Get - לכשאכנסנה אגרשנה אינו גט
Summary:
A woman may remarry after receiving a גט ישן (provided there was only יחוד); however if there was ביאה it is an invalid גט, unless it was dated for after the נישואין.
Here too, it is an enactment; Perhaps they will say her Get preceded her son - הכא נמי גזירה שמא יאמרו גיטה קודם לבנה
Summary:
עדים will sign a pre-dated גט אירוסין (even after the נישואין), for they will assume that she is still an ארוסה (so there is no פירי issue). An actual divorce date will insure that there are no problems of פירי and גיטה קודם לבנה.