All punctuation in translations is somewhat irrelevant, as the oldest manuscripts had no punctuation or accents whatsoever and in many cases not even any spaces between words. Although Greek texts like the Nestle-Aland or the 1904 Patriarchal Text of the Orthodox Church have punctuation, it was placed there by the latter text editors and is not something found in the manuscripts.
The Greek word for devil in Revelation 20:1 is διάβολος (diabolos). In the majority of cases in the Septuagint (18 of 20), διάβολος translates the Hebrew שטן (Masoretic שָׂטָן - śā·ṭān), which in addition to being the proper name for the devil also means adversary. Satan in the KJV is the English translation of Σατανᾶς (Satanas), which is nothing more than the Greek transliteration of the Hebrew word. Therefore, I would say that Devil and Satan are understood to be equivalent in this case.
The Greek text - τὸν δράκοντα τὸν ὄφιν τὸν ἀρχαῖον ὅς ἐστι Διάβολος ... - includes the relative pronoun ὅς, so the KJV translation is accurate here, I think. The dragon (δράκων), serpent (ὄφις), devil (διάβολος) and Satan (Σατανᾶς) all refer to the same thing. At least Greeks seem to have understood it this way. Andrew of Caesarea (563-637), author of the oldest complete commentary on Revelation by a Greek Church Father, did not see multiple entities in his commentary:
Here he narrates the destruction of the devil which had taken place during the Master's passion, in which he who appeared to be strong, having bound us (as) his spoils, [Matt 12:29] one stronger than he, Christ our God, redeemed us from his hands, condemning him to the abyss.*
* Translation from E. Constantinou, Andrew of Caesarea and the Apocalypse of in the Ancient Church in the East: Studies and Translation (Université Laval, 2008)