The speed at which Henry comes to power is a stretch, yes. Until you realise just how fast you can become a good enough fighter to survive a battle if you don’t rush headlong. And if you have the proper armor. That said, it is still a stretch until you finish the main story and then everything makes more sense.
A female would be a great companion or supporting character, but otherwise would fail to be enthralling for 90% of the people who would buy a game like this. It would require way more work or effort then what it is worth to warhorse and to many of the fans. Lady Steph is manipulative and amazing, yes. And so is Theresa, but lets be honest. Most people would not enjoy their stories as a playable character and it would honestly not be worth our time playing due to the heavy restrictions placed by the time period.
The only realistic ideal is a lower born noblewoman manipulating a male relative or her husband to gain greater power for herself and family.
And, lets be honest, that would make a great book but a bad video game and bussiness descision. You have other options to rp a strong female character in a medieval like era. Mount and Blade bannerlord will be one.