"c" being pronounced as "ts" would be familiar to most people from Central or Eastern Europe, for example. Even coming from English, "c" can often mean "s" which is at least similar.
I'd say that the most unusual pinyin mappings are "q" and "x", although both have some analogues in European languages as well.
X is probably familiar to most people because of Xi Jinping's surname. I agree with the notion that Q is the Pinyin letter that would cause the most trouble for Westerners.
I'd say that the most unusual pinyin mappings are "q" and "x", although both have some analogues in European languages as well.