The Protoevangdlium is not scripture, but it is evidence of the Jewish culture. If the notion of a custodial marriage were as foreign ro the 1c Jews as it is to us, it would not have been copied through, considered for canonization and generally treated as reliable before veneration of Mary became wide spread.
It may show some of Jewish culture, but it shouldn't be used as evidence of anything doctrinal any more than the many non-Scriptural books coming out at the time should be considered for doctrine seeing that it was NOT inspired Scripture.