Salem

Okay, I've finally got around to paying attention to slotting artifacts. The problem is, I think every attempt resulted in a broken slot. I can't find out exactly what that means, other than I assume the attempt failed. But does that mean I'm screwed from ever slotting anything on that article of clothing? Will I be able to try again after a period of time, or is there something I can do to fix the slot? I'll be very sad if not, as I think every slot of mine is broken currently.

Yep. Once you break a slot, you can't slot anything else there. You have to replace the article of clothing if you want to try again.

Even with perfectly matching proficiencies and reasonably "easy" artifacts I always get success % below 50%. I just think there is something wrong in the calculation or you simply need insanely high proficiencies

Some artifacts I could even NOT find a single cloth with matching proficiencies (e.g. Feathered Brooch). That is why I give up at the moment.

That is the risk if there wasn't any then everyone would have very powerful equipment - someone who is lucky cant have an advantage over another who has better biles, I feel it evens the playing field a bit.
Also it is a good idea to slot 1 or 2 slot items, that way there is less risk and try for greater slot items when you have many snakes skulls to burn later on.