I just received the expansion and read the rules.
The rules state to remove 15 cards prior to adding the expansions cards. Whats the rationale behind that?
How is this different from just adding the expansion cards?
Also: can I easily distinguish the expansion cards from the original cards?
It is only to give you a better chance of getting one of the new cards while you play and also, if you add the broken wands back into the deck, it gives the same chance that the two wands will come out as in a normal game. Mainly, it's about getting the new cards more chances for showing up.
The backsides should be colored differently enough that they can be easily distinguished, as they were printed by different printers at different times. Different backs have very little impact, if any, on the play and decisions.