Translation:Yes, you may
Shouldn't it also accept "You may" as an answer? I know in English we tend to put the yes in there as well ("Yes, you may"), but "cewch" is literally "you may."
It could be translated as "Yes, you may". So "Ga i mynd i'r siop?" (Can/May I go to the shop?) would be answered with "Cewch".
I believe so, so "Cewch chi" and "Cei di" (don't know why the "T" has mutated there but it just feels right)
Probably due to cei ending in a vowel – which is the original cause of mutations, though many vowels were later dropped but the mutations remained, which is why adjectives mutate after feminine nouns (most of them used to end in vowels)
You may already know that or not, but I fear we are running out of reply levels so clarified just in case.