"Apparently there is no money attached to indulgences anymore, however, how did the RC Church wildly spin this as doctrine? Where did the authority come from?"
Alright, there are three pieces to this, and I'll give them in order.
First, the idea that the prayers and offerings of the living can help the dead to atone for their sins is in 2 Maccabbees. There, you may recall, is recounted the story of the Jewish warriors who were all slain, and were found to be wearing medallions to some goddess. One of the Maccabbees offers up prayers and offerings to God so that this sin of idolatry, for which these otherwise loyal soldiers had already paid with their lives (by being defeated in battle and killed) would be blotted out by God and not held against them in the afterlife. This is the first place in the Bible where the doctrine of the resurrection is specifically mentioned by name.
So, that is where the Biblical authority comes from that tells us that prayers and offering for the dead help them before God.
Secondly, where the Biblical authority comes from that allows the Pope to issue decretals such as the one you cited, which systematizes everything, lies in Matthew (etc.), in the power of the keys, to loose and to bind, granted to Peter by Jesus. Paul (Timothy) tells us how authority was passed to successors by the laying on of hands. And that is why the Pope has the authority to do things like the document you cited: the power of the keys, passed to him in apostolic succession.
The third thing is my own opinion: THIS is the sort of thing that just makes me cringe and groan. It's just TOO regularized, systematized, rational, and logical...it has the feel of something that was made up out of wholecloth, defended vigorously at some earlier point in history, which then has to be suffered as a "tradition" today.
Now, were the specificities concerning indulgences laid out by a Saint who had talked to God and angels and said so - there have been many such divine messages and revelations delivered through Saints, it would be easy to swallow.
But this...well...it just has the feel of a government bureaucracy administering drops of salvation, doesn't it?
And it's not very credible, is it?
THIS is precisely the part of the dead wood in the Catholic tradition that really needs to be chopped out.
The big picture is true: prayers and offerings for the dead are good things, they are beneficial, like prayers for anyone else. The traditions, including the Bible in 2 Maccabbees tell us so.
And that's where we should firmly stay: in the big picture.
When you start getting down into pounds and pence and how many ounces of forgiveness a given prayer or votary candle is worth...well...it's embarrassing that the Church used to do that, and we need to knock that stuff off today.
In my opinion.
We're on the same page here. Indulgences are largely ignored by most Catholics, including clergy, but we just had the Pope grant a plenary for some reason or another the other day.
These practices are part of a single-file salvation that encouraged each Catholic to focus on his own redemption, almost to the exclusion of our brothers and sisters.
Wrongheaded, IMO.