Shouldn't it be "Ces sont" and not Ce sont?
"Ces sont" is always changed to "Ce sont" because of the collision of the two S sounds. Another way to say this is: C'est becomes ce sont when followed by a plural noun. It's just one of those things we have to remember.
'ces' is not used on it's own, it's an adjective like 'cette': only used in addition to a noun.
Not really. "ces" is the plural form of adjectives "ce", "cet" and "cette". The plural form of pronoun "ce" is still "ce".
Would a female prisoner be 'prisonnière'?
yes, for pronounciation:
but it should be prisonnières if you are going to put it in the lesson, though the pronounciation is more for male.
Very helpful sentence