First of all, that 拡げられる is not a potential form. It’s the passive form of 拡げる.
Secondly, calling that 〜ている “progressive” is misleading. It describes a state that has resulted from some earlier action or change. 拡げられている describes such a state in the present, whereas 拡げられていた describes one in the past. Although I cannot tell for sure because the main clause is missing, I guess the narrator is describing a state at some time in the past.
とく (解く) is neither potential nor progressive. It is a transitive verb that means “to untie” or “to unpack”. 包み is its object here.
I think part of the reason you find this sentence hard to understand is that とく is used in the active voice while 拡げる is in the passive as we have seen above, yet those two are connected with a て-form. Since the subject is 見本, it does sound like the samples unpacked their own packages (actively) and then (passively) be spread out (to stay in the resulting state). 見本が包みをとかれて拡げられていた would have been better balanced and might have been easier to understand.