The redundancy comes in part from the oldest forms of the Latin alphabet, where there were 3 forms of C, K was used before A, C was used before E or I and Q was

The redundancy comes in part from the oldest forms of the Latin alphabet, where there were 3 forms of C, K was used before A, C was used before E or I and Q was used before O or U. Later they have understood that writing with different letters minor variations in pronunciation of the same phoneme is useless, so they began to use C in almost all cases where previously either K or C or Q were used, but K and Q have survived for some niche uses. The next complication has arisen in the Middle Ages, when in many European languages the pronunciation of C before E and I has been changed and instead of being the same as K it has become like the pronunciation of either English "s" or English "sh" or English "ch" or English "ts". Because English has borrowed many words from a lot of languages, keeping their spelling, it has gathered all these pronunciations, so as you have pointed, all these different pronunciations are possible in English and it is unpredictable which of them must be used in a given word.