I can show some respect to his reasoning... but good reasoning is dime a dozen.
Sending your family and soldiers to the slaughter because of good reasoning and achieving great victory because of petty desires are on the same level of prideful bullshitery stupidity to me.
I feel like Shizuko knows he is Nagamasa, she just doesn't have "confirmation" on his reasons, so to speak... which brings me to... quite ballsy of our girl accepting sending this guy with the assault if she is uncertain of the possible developments.
At this point, Shizuko clearly has accepted that they have already completely deviated from the "normal" timeline, and she has steeled her resolve to see this life through this.
But that doesn't mean she has complete control over everything, and that uncertain factors can't just pop out, especially because... we are talking of a timetable that can span decades, there are many changes already, many "characters" that shouldn't even be alive at this point anymore, and others that should be alive but current events may change how the act in the future, and so on.