QI have 2 copies of this follower on my field. If my opponent plays Themis's Decree and destroys both of them, do I summon 2 Aftershock tokens?

ANo. When your 2 copies of this follower are destroyed and put into the cemetery, both of their Last Words abilities trigger and become pending at the same time. You then resolve them one by one.

When resolving the first Last Words ability, there's no Aftershock on your field, so you summon one. When resolving the second Last Words ability, there is an Aftershock on your field, so you place a lightning counter on it instead.

QThere's no Aftershock on my field when I play this follower's Last Words ability, so I summon one accordingly. Do I then place a lightning counter on each Aftershock on my field because there's an Aftershock on my field now?

ANo. Aside from selections made as a prerequisite to play an ability, abilities are resolved in the written order.