The first is impossible to say, even for a native speaker. You cannot directly follow a 'ch' sound with a 'z' sound without adding an extra syllable. The usual pronunciation of both 'witch's' and 'witches' is this : /wɪtʃɪz/

你好Kyle, The first pronunciation you listed would be acceptable, but not best. The second pronunciation, again, would work, but it is the pronunciation for 'which is'. You may want to add a schwa to the first pronunciation to make /'wɪtʃ.əz/.

The second - if you have a word ending with /cz/ or /sz/ the final sound in plural is /iz/ Bob