When the "subject" takes ने, the verb agrees with the "object" (elephants). Since that's plural, it's देखे.
Yep but in Hindi you can often use the singular version of the noun even if you are meaning plural, not sure if that's 100% grammatically correct, but everyone speaks like this. Then the way you tell whether they are talking about singular or plural elephants is the verb at the end, here देखे, so plural. हाथियों would also be correct though.