While its a bit unfair to judge Giren since we're basing it on game Sera's memories, current Giren is just a bit overbearing but still has some humanity left. I will agree him proposing to a 6 year old (no matter how mentally old she is) is absurd but I guess he's doing this to avoid as much competition as possible (I mean, they're against a foreign crown prince), not like current Sera has any chance of marrying a noble since her rep is still no magic girl.
I am open for the ship so long as their fluffy scenes are just... that. Simple and fluffy like Roux. I can accept one kiss on the cheek as a form of teasing at most. 2nd ML also isn'tany better since the gap is still 8 years (Giren is most likely a year older), I'm basing all of this in Laruzza's manga age (13) mentioned as early as chapter 1/2. His novel age I believe, he (Laruzza) and Sera only have an age gap of 4 years(? I may be wrong though and the manga was right. The one I'm sure of is that their mom died after giving birth to Sera and Laruzza already knows the concept of death and loss for his age. Enough to cause a misunderstanding where the manga just skips im its adaptation). He's still a first year mage, that's for sure and CP and Giren are at least 2nd and 3rd years respectively.