I want you to imitate what Mouten just did and figure out yourself just how impossible it is to pull that stab off while simultanously falling off of your horse - especially when your weapon is shorter than the one that you just blocked.
It's bullshit... but I like it.
Duke sees Stella only as his little sister but she clearly wants his dick, so obviously she would make all the staff to hate the Duchess. And the Duke is just overprotective and trusts everything Stella says. And since she's handicapped, it might be too hard for her to travel, which explains why...
This would be believable if it wasn't fucking Japan where practically no crime ever happens - and where crimes get solved the most often.
But EH, stories always need bad guys...
Sexual relationships outside of marriage are verboten! And that means no hand-holding either! They must at least get engaged first. This manga is too indecent for young and impressionable children like us!