I can try to answer a few of your points if you want.
Not sure if your only suppose to get 3 people in a party, that is all I could find while searching the whole island.
There are 4 npcs you can ally yourself with in Fort Joy, but you can only have a maximum party size of 4. The Red Prince on the beach at the very start contemplating suicide. Lohse praying to the Shrine of the Seven. Ivan-something next to the necromancy vendor. And Sebille sneaking around Stingtail.
Librarian and Pigs - Bless doesn't seem to work on removing the necro-fire. Wasn't able to figure out a solution to this.
For the Librarian, you have to get him on a puddle of blood and bless it. (May take multiple tries) which will cure him permanently of his condition. You can use Flesh Sacrifice with an elf and bless the resulting blood puddle.
As for the pigs.. they are somewhat bugged. If you wanna make sure to bless them correctly, just talk to them one by one with one of your characters, and bless them repeatedly until you see the 600xp gained popup. You will have a surprise once you bless all the pigs.
Squeaky Ball - not sure where I found this or where to use it. I wish I could take it to work so I can annoy everyone in the office. Guess I'm stuck having to spend money on a real one...
You can choose to use that ball on the hounds (on the first floor of the Fort) to avoid fighting them.
