Yeah I had a similar question. I'm looking to play with powers next time.
So the shards have to be placed from bottom up right? So if you want to use a power like philosopher's stone that says it must be placed on the 3rd or 4th slots you must have already placed 1 on the bottom slot and 2 on the second slot and 3 on the third slot for 6 placed shards. Then you can learn that power right? Even if you don't have powers on the bottom two?
Next round I could learn another power for one of the bottom two slots right? Iebits possible to have 4 powers going at once yes?
Also, when you spend shards, you first spend any you happen to have that are not on the slots, and then you spend from the top down.