As far as IMDB is telling me, none of the writers from Bioware had anything to do with DTOS, so I'm not sure why people even use it as reference for anything.
BioWare call it canon.
It's mentioned a few times in World of Thedas.
It's mentioned on the official DAI timeline too.
The Cassandra in it is remarkably constant with her "character profile interview" except that she's younger. It is remarkable, because most of the time 3rd party adaptions are craps in regard to characterizations.
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As for Cassandra, Galyan and the kiss on the cheek. She also gives a kiss to Byron in DotS and he is a father figure to her, not a love interest. Seems like Cassandra is just the type of person who don't mind to showing affection to friends/family (in private, both case had only her and the other person in the room).