Alright, I'll try to clarify as much as I can.
First off:
The Drakefire Amulet quest, which is the end of the quest line listed in the first post here, is what attunes you for Onyxia's Lair. When you finish it, you get the Drakefire Amulet, which must be in your possession, but not necessarily worn, to get into her Lair. Every raid member must have this item.
Second:
In order to finish the last bit of that quest line, you have to go into Upper Blackrock Spire and kill the end boss. This gets you what you need to have the quest giver make the Drakefire Amulet.
Third:
However, in order to get into UBRS, at least one person (I believe only one, though I may have to double check that) must have the Seal of Ascension ring, the actual item, that Det mentioned above.
Fourth:
To get the Seal of Ascension is where the gems mentioned come in. In LBRS, a Unadorned Seal of Ascension item drops, and each of the four(?) main bosses drops a gem, each of which must be in the players possession for the Vaelen guy to make the Adorned Seal of Ascension, which then requires a few more quests to turn into the Seal of Ascension ring.
Note: In order to turn in the Adorned Seal for the Seal ring, you (or possibly someone in your party) must have another Unadorned Seal of Ascension in their possession, or else Vaelen won't even talk to you.
So...:
The Seal of Ascension ring is the 'key' to get into UBRS.
The Onyxia quest chain finishes with a run through UBRS.
The Drakefire Amulet from the Onyxia quest chain is required to get into Onyxia's Lair.
And to answer your final question, the run this Saturday will be through UBRS in order to finish the Onyxia quest chain and get the Drakefire Amulet. It's extremely unlikely that we'll actually go against Onyxia herself then. Since UBRS is a 10 man raid and Onyxia is a 40 man raid, we'll have to hit UBRS at least a couple times to get enough people attuned.
Please be ready for the UBRS run by completing as much of the Onyxia quest chain as you can. You must at the very 'least' have completed the BRD section of it to join in the group.
Also, yes, I do have the Seal of Ascension ring for getting into UBRS. So as long as only one person needs it, we're fine there. However, since UBRS has to be run more than once, as more than 10 people will likely want to go to Onyxia, and likely be needed for her, it certainly wouldn't hurt more people to have the Seal of Ascension.