Not really. In societies on the level of advancement of the HC series nobility was typically reserved for land owners. Like if you were going to ascend to nobility you had to at least be among the gentry first and unfortunately Leto is a poor orphan. Rose probably should have had her father knight Leto so at the very least she could have stuck him into her entourage.Clearly whole mess would've been avoided if she just ennobled Leto
Although let's be honest... if she's been sneaking away from the castle for a long enough period of her life for them to actually develop a romantic relationship then they should have just started fucking at some point. If they'd already been in a sexual relationship with the duke showed up Rose could have just yoinked the receptacle for her energy from the duke and rushed over to Leto to go for a ride.