The only real way to be an eighth elvish would be to have a single elvish great-grandparent, which means... *digs for LoTR appendicies* ...said agent would have to be a child of Vardamir, second king of Numenor (and thus entirely Numenorean herself), or alternatively... *digs for Silm* ...a great-grandchild of Idril and Tuor? She would have to be a grandchild of a sibling of Earendil's, and given the human requirement, that would put her lifespan closer to the fall of Gondolin than the fall of Numenor.

In other words, canonical pairings are pretty much out of the question. You will probably have to invent one. Considering that the Valar basically said "no more" when Elrond and Elros decided, I rather doubt there would be another...

The trouble with Tolkien's universe is that Elvishness really isn't very divisible. If you're set on having a part-elvish Agent, you might want to pick a more condusive Canon for her to come from.

1) Universes don't get PPC'd - fics do. The Spies who trawl for badfic submit reports when they find fics that require PPCing. This is what the Department of Intelligence does.

2) Only if they're an ex-Sue or semi-fic blip, I suspect. Being part-anything, part-human tends to be considered Sueish, and you'd need a good justification for doing it.

3) Depends on what works best for the story. If it would be handy for them to be bit characters, then treat them as such. If they would only get in the way, send 'em to the CAF agency, pretty much. Unless, despite not being able to talk, they display definite intelligence, in which case they'd be bits, I suspect.

4) Do you mean a typo in the original published book? It might create something, not necessarily a mini.

Disclaimer - things I am not include an authority.

I'll answer question 4.

Possibly. It depends on whether fanfic writers use said misprint as canon in their fanfics. A good example is in the Official Fanfiction University of Redwall, where one of the students read an earlier edition of Redwall, which had a misprint that Killconey the ferret was a girl. As a result, said student was responsible for the creation of Girl!Killconey, or Konnie as she is often referred to.