しかも is pretty straight forward:
今度、「一番」ってお店のランチに行こうよ。あそこ、すごくおいしんだよ。しかも量もおおいよ。
Let's go get lunch at Ichiban next time. The food is great, also the servings are big.うちの子は天才だよ。ピアノを弾くのが上手だ。しかも先月、英検2級を取った。
My kid is a genius. He's great at playing the piano. What's more, last month he passed the 2nd grade of the Eiken.
I think of しかも as 'Also', 'What's more', or 'On top of that'.
The thing is I don't see how that is more different than それどころか. For example:
この間食堂で食べた親子丼はひどかった。まず、にくはちゃんと焼いてなかったし、それどころかごはんは少なかった。
I had a oyako-don at the dining hall and it was terrible. To begin with, the meat wasn't cooked properly, what's more, there wasn't a lot of rice.
I have a feeling that the translation of the two phrases is not going to change very much when put into English, but that is not really important. What I want to know is the correct time to use each respective phrase. Or, are they not as interchangeable as I am thinking?
Thanks!