Are there rules defining what qualifies as "true love" etc. in game?

I would say that your true love probably deserves an Aspect devoted to it (check out Thomas and Justine's writeups for an example).

Fully fae means you don't have free will. You are bound to your nature. If they meant for there to be an option to be fully fae and be a PC, then there would be a Fae template.

If the Choice was meant to make them go NPC, then they would be specifically noted (like the Red Court Infected on page 80).

Keep in mind that most of the fey we see Harry dealing with are those in the upper reaches of power: Mab, Maeve, Leanansidhe.  That's some serious refresh cost there.  And one changeling we saw go fey became the Summer Lady.  There's a lot more leeway at the lower levels - when Morgan is ready to kill Harry in Storm Front for breaking the Fourth Law, the argument is meaningless if Toot doesn't have free will (or something like it).

Last, in the game, the idea of "free will" is a sliding scale.  A White Court Virgin in a "Feet in the Water" game who kills using her Emotional Vampire abilities becomes an NPC and loses all of her free will - but the exact same character in a "Chest Deep" game is a viable PC.

The rules disagree with you.  Pg. 75 YS

I don't see anything that looks like "becomes an NPC" on that page.  However, the page before says "often becoming an NPC as his power costs rise to exceed his refresh", which suggests that the usual rules for crossing the line into NPCdom apply.

I'm also not entirely convinced that no fey have free will.  The Wyldfae seem to have some element of it, in that they can choose between Summer and Winter, and Harry scrupulously avoided using compulsions on Toot (Morgan tried to come down on him about it - sure, he was looking for any excuse, but he seemed to think he might have had a case).

One of my other aspects is: "Dammit I should never have accepted that bargain" indicating (i hope if i´ve understood aspects chapter yet) that I´ve made a deal with one of the sidhe and that the creature can still collect a favor of me. So my GM can integrate it if he wants to.

Does it hold up? could that be a real aspect or have I missunderstood or not read far enough yet?

First, I want to just state my amusement at seeing a post asking "have I gotten in over my head?" from Nicodemus Archleone.

Second, make sure your GM understands what you are looking for with that aspect.  If I had to guess I would think that the character had anentire history of unwise bargains - maybe he is too eager to trade open favors for short-term success - and I would compel liberally to continue that trend.   "The man straightens his noose and smiles, placing a tarnished silver coin on the table" - that sort of thing.  Which is great for story and I would love it as a GM if someone did this.  I would totally see that character as making an escalating set of deals to get out of the previous ones, trying desperately to finish before the whole house of cards collapses.

If you want it to be a specific bargain you are regretting, my suggestion would be something more like "Never bargain with a faerie queen" or even "In Titania's Debt".

Diaspora gets away with adding stress boxes via stunts, but damage is much nastier in that variant: you check off the affected box and all unchecked boxes below it. It makes an additional box mean a lot less.

Alertness, to scout out a spot where you won't be disturbed
Athletics, to climb out onto the roof ledge of a building and get a pigeon's feather (maybe for a long distance communication spell?)
Burglary, to get access to something of your target's
Contacts, reaching out to another wizard who specializes in this sort of thing
Conviction for the actual casting
Craftsmanship to create the ritual materials you will need
Deceit to convince the target to tell you their name (from their own lips, it has power!)
Discipline for the casting - and staying up late nights researching the spell
Driving because you are out of candles and the store closes in 5 minutes
Empathy to learn an Aspect of the target you can use to help guide the spell
Endurance for those long nits of research - or maybe you need to put blood into the spell
Fists or Guns when someone discovers you taking a spell component that you shouldn't
Intimidation to stare down a summoned spirit and browbeat it into giving you information

Got to get back to work, maybe someone else can finish out the list!

6) Even very simple thaumaturgy spells take at least 15 minutes to complete unless you have a sponsor that speeds it up to evocation for you.  It isn't specified who long any specific spell takes to complete, but assume that the higher the complexity, the more time it takes.

Not quite.  Check out YS270-271 for the timing rules.  If you're not under stress, you're just assumed to cast the spell once you have all the preparation (if any) completed.  If you are under stress (there's a frog demon trying to batter down your door) that's when you have to cast it "quickly" and you start making Discipline rolls. Then the casting time becomes measured in exchanges.  In the example, Harry does a Thaumaturgy spell in what is effectively combat time (3 rounds).