I don't think per sé that Harry felt the magic was wrong for him in Storm Front. But he didn't want to risk being associated with it. pspoiler]Remember that the Doom of damocles was still over him in that book.[/spoiler] He didn't want to get in to trouble with the wardens and it was black magic.
Now referring to changes where Harry borrows staff. he recognizes the item as familiar because his own staff was similar made from the same tree and under the tutelage of his master Ebenezer.
which was why he could use it.
I would suspect that another focus item that Harry wasn't familiar with wouldn't fit him so well and he wouldn't be able to use it effectively. Probably not before he had a chance to familiarize himself with how it functioned through study and practise.
I just started to read the Grave Peril case file, where Harry is giving his wristband to this "Lydia". So, it seems to be fairly easy to give some item of power to someone else to use it for more than a scene, or is it not (I'm just at the beginning of the book). The question would be, is it still Harry's wristband (and occupies one of his item slots) or is it now hers (and occupies one of her item slots?
Lydia was a minor talent with Cassandra's Tears. Rules wise she does not have Items slots.
So, the Wristband is still Harry's, and it still occupies his slots. He obviously made it with the extra slot that let's him loan it out to other people. He can loan it out and get it back again.
This loan is not restricted to one scene. Theoretically it can go on for a long, long time.
You could of course say that Lydia has her own slots, and that Harry permanently transferred the item to her, in that case there would have been no need for the item to have the extra slot, but Harry would not be able to get the item back. However, his slots would be free again.
Which leads us to the next question.
By the way: I'm I right that items are only usable by people that have item slots? So, a pure mortal could not use any item?
If an item is made with the extra slot, then anyone can use it. Even someone without slots, even a pure mortal.
Examples include Lydia, who has no slots of her own.
Thomas, who used Harry's duster and also has no slots of his own.
Buzz, from "The Warrior" who had a protective amulet,
and is a pure mortal.
Items made without this slot are only usable by the creator. Or their "adoptive parent".
Potions are another matter, they are usable by anyone, without need for an extra slot.