I should know better than to ask two questions in one post.

So who has figured or think they figured out what it means to train a player with a set that only has one skill applicable to the player and four that are not?

Because there are plenty of skills (particularly for GK) that are only available via such sets (some of them listed under Attack), I am proceeding under the assumption that the player in this case gets full advertised benefit of the set, all applied towards one applicable skill. Anything else would be counterintuitive and unfair, but hey, this is Nordeus.