Completely agree with every word of your post. Not surprised by your anecdotal stats.Okay, let's see.
1) Those who read books and played games before already have an opinion of both and would choose independently of how the game represents Yen and Triss.
2) Those who only played games have a complete opinion of Triss and they know that Yen used to be Geralt's love for long. If they are Triss fans already - they would most probably choose Triss. If they aren't or they doubt - the game heavily implies you should choose Yennefer. The dream about her and Ciri in the very beginning, in Vyzima she already talks to Geralt like she would do with her lover, then comes Priscilla's song and a lot of tips on Skellige, Kaer Morhen etc (though Skelllige and further might be already too late to choose). If they chose Triss on their first playthrough, they will most likely try Yennefer on their second playthrough.
3) Those who didn't play games either would choose according to their first impression of both. Still, their opinion would be affected by a lot of tips about choosing Yennefer. In fact, only Dijkstra acknowledges Triss and Geralt as lovers, while there are a lot more implications about Geralt and Yennefer.
I talked to my friends about this a bit. My personal experience says this:
2 persons (including me) read books, played games, chose Triss, 2 read books, played games, chose Yen.
2 only played games, one of them chose Triss, the other - Yennefer.
3 only played TW3, all of them chose Yennefer.
It's not representative, but still shows something.
The large reddit poll on the issue came back with 51 to 49 % Yen over Triss if i recall.