I have no idea how PSS gets there. As you say, and as all the translations seem to indicate, the word used here is in the plural. 'Christ died for our
sins, according to the scriptures'. This word is often ued in the plural. If there was a singular concept ('Christ died for our sin') then there were other ways that could have been expressed. Take for example in John 1:29
Ἴδε ὁ ἀμνὸς τοῦ θεοῦ, ὁ αἴρων
τὴν ἁμαρτίαν τοῦ κόσμου.
The same word is being used but in its singular form. People use the same words to mean different things at different times or to add different kinds of emphasis. Obviously John is stressing the condition of the world as a whole and that Jesus came to deal with this condition
as a whole. Whilst Paul and other writers were looking at it from the individual's point of view.
I do not believe that Calvinistic predestination of all things is compatible with freedom generally.
In response, Calvinists will typically quote reams and reams of scriptures demonstrating God's sovereignty. All in vain, as that is not the issue. Sovereignty does not equate to predestination. Calvinists limit God by their teaching that in order for God to be sovereign he must predestine all things. God is clever and capable enough to do it differently. In fact it doesn't take much lateral thinking to see how God could manage the world, achieve his plans, without having to predestine every last things down to the smallest atom. Calvinists lack faith as well as imagination. Rather, his sovereignty is manifested in the freedom he gives to the world, which implies that the world can live.
Predestination is incompatible with life itself (a big read but if you had one last chance to say everything to everyone you know, I am sure you would not limit it to just 15 words!).