While I agree it's a valid alternative, I don't totally agree with your pro's and con's.
While an 18-yo trained in his primaries is better suited at playing league games, that difference isn't making a difference in his market value. People are not paying 500k for a 19 year old MVP to just play league games. The people paying the price you get either way, are people that are going to train him. I don't think his value will rise because he can play league games better. If anything, I think his value will rise when training secondaries because teams don't have to train him out of position that much, so he'll be better in league games.
A difference in our take is that you suggest to sell after 1 season. I was thinking more of a 3-5 season window, in which he'll have a reasonable salary when trained in secondaries.
If you intend to sell him after 1 season, training primaries is a good thing to do. If his secondaries aren't terrible, people don't mind him being trained just in the primaries, and it will increase his value for sure. If you intend to sell him after 3-5 season, it's better to start training secondaries first, because after 3-5 seasons people cannot really train secondaries anymore, and by then he'll have lost potential value.