I don't think neither the light spirit nor the heroine know about the 'charm' effect of the healing light.
The spirit will try to show Cecil the 'alternate realities' where he doesn't end up with the heroines, none of them with good results (even the warmonger one) to convince him that he NEED to pick the heroine to avoid becoming like that.
But pretty much all of them, Bertia included, misunderstood how it actually work.
What'll prevent Cecil from becoming an 'android prince' isn't a fated maiden, but ANYTHING that spark his interest.
When he finds one, he'll relentlessly keep to it because it's the only thing that makes him feel alive.
But when it's not a person, he follows that interest to the extreme until he goes too far which is what all those alternate routes show.
And yes, Bertia already did trigger his interest on their first meeting, so heroine had 0 chance of succeeding anyway because he already found his 'fated maiden'
As for the mark thing, it's a spirit thing, and Cecil will totally put it on Bertia just to convince her SHE is the one (Kuro will play along albeit not amused at having to trick her own master)