You can replace as many as you like (pg.79 malign sorc), only thing that would stop it is specific tournement rules that you can only have 1.
Normally you can give allies items of power (no rules against it just a hero in your army), but the cave sham's badly written FAQ means it is heavily up for debate.
"Lord-Ordinators, Darkoath Warqueens, Knights of
Shrouds and Fungoid Cave-Shamans can be given a
command trait or artefact of power if they share the
keyword of that army’s allegiance."
This is silly for artifacts because allies can normally take them, but makes sense for the command traits because they are alowed to be the general without being of the same alleigence. I say baddly written because it should either have the word only in there, or say you can't do it.
Edit: Endless spells are one attempt of any endless spell per wizard per turn