Au contraire, sir.  Ask any member of Hell's Angels, they will cheerfully tell that a tattoo can be removed.  It can be flogged off, burned off, etc.  Tattoos can quite assuredly be removed.

Yes, but that removal isn't something that can happen just casually; you don't check your tattoos at the door the way you could a gun or a staff or a jacket.  Thus, a magic tattoo isn't an item of power; it's just a power.

I'd tend to say, if the supernatural someone still thought of the place as home (and their family still lives there), they could get in.  This would, imo, immediately rule out DF Black Court vampires - they're sufficiently inhuman that the concept of "home" doesn't work for them.  A White Court vampire, on the other hand... though, of course, the WCV could walk through an active threshold with no problem; it would merely limit the amount of supernatural power they could bring to bear.

In general, I'd say the creature needs to be something that is capable of generating a threshold before it could claim free entry on the basis of somewhere being its home.  So, probably not Red Court vampires, either.  Scions of most types, sure.  Dragons (even those with a lower case 'd'), sure.  Kitsune (but probably not kumiho), maybe tengu, definitely not fae (Though they get their own exceptions...), and so on and so forth.

I'd model it as a power that requires an associated aspect, and gives you the ability to make a relevant declaration once per scene, for free.  Cost then depends on how relevant its scope is, and how many free declarations you get.  A place like Demonreach might give, say, two free declarations per scene, at the cost of one refresh - since most of the time, things aren't happening on the island anyway.  Shagnasty's pain infliction sense might cost more like two refresh for one declaration per scene, though, since it will always be using it.

Hm.  I may have to take a stab at this.

In the meantime, though - I'm not sure pinning this thread is actually helping it; I know I at least don't even look at the pinned threads when I'm trying to see if there's something new & interesting; only found it because I was thinking "Surely someone has posted a link to the contest already," followed by "maybe it got pinned?" - if I hadn't already known the contest existed, I would never have found this thread.

Another power that is potentially abuseable is beast change.  An aggressive player can construct their character in such a  way as to have very good skills in all areas and steal the show from the other players.

I'm not sure I agree - the proposed character is certainly very strong... but only when they're prepared.  A simple mugging could take out the human form before he got a chance to shift, and it's not hard to imagine fights where going animal form just wouldn't be acceptable (for example, against vanilla mortals with cameras), while the animal form would be very vulnerable to intimidation or other mental attacks.  In fact, hit them with a typical White Court vampire, and they just lose - either physically stomped by the WCV's always-available powers, or psychically stomped if they try to go animal form.

It's definitely competitive with a wizard... but I don't think it's more than competitive.

Things to watch out for: focused crafters - while you can't take rituals and then add refinements for specializations, you can add refinements for focus items... and focus items can improve enchanted items.  The results can easily be absurd.

Thaumaturgy in general - I'd strongly suggest setting specific expectations for time & effort required to accomplish various power levels of rituals, so you don't get some munchkin wandering in and assuming that they can spend half a day making discipline maneuvers of "gathering power" (or, for the creative munchkin, invent 15 different aspects that all completely coincidentally use the character's apex skill to create) and then toss off a thirty shift ritual of doom.

Evocation is - at the refresh level you're starting at - actually much less dangerous, mostly due to the cost of throwing around high-power spells.  However, if people can advance to, say, mid-teens refresh, evocation can get rather ridiculous as well.

Another thing to watch out for is stunts - there's some disagreement over how limited a stunt needs to be, particularly in regards to making attack & defense bonuses.  Skill swap stunts can also be dangerous - a typical munchkin move is to find a way to skill swap all of their attacks and defenses to a single apex skill, then add stunts that boost that skill in a "limited" set of circumstances... that, on closer examination, turns out to not be actually limiting.
A really obvious example of an overly broad stunt might be "+1 weapons skill when in combat", though most munchkins will go with something subtler - like "defend with discipline when I can see the attack coming", which seems fairly reasonable for a wizard until you consider that, one, if you can't see the attack coming you automatically defend at +0 anyway, and two, physical defense is really about two-and-a-half trappings (defense vs. melee attacks, defense versus ranged attacks, and the one-half is defense versus unarmed attacks - which is included in defense versus melee, but can also be granted on its own, e.g. by the fists skill.)

Alas, much of this advice can be boiled down to "don't let players make hyperfocused characters", and "watch for munchkins", rather than any easily-house-ruleable flaws in the game system.
Oh, and look really carefully at any 'refluffed' powers, or powers taken from OW; while these are often just fine in their original context, many of them are very easily abused.  See, for example, the most recent thread about A Few Seconds Ahead - a power which I'd strongly suggest explicitly banning; while it's okay on a low lore character, it's just silly on any of the PCs that would take it.  There are better ways to work the mechanics of a precognitive power anyway.

Alternatively, you could go with more of the original mythos, and have the wail not be a killing attack (D&D popularized that), but instead be a notification that someone has died... or is fated to die.  In the latter case, you could run it a bit like the barabbas curse... or you could run it as a warning, and say that - if the hearer heeds the warning - free will allows gaining a bonus versus death.  (I.E. free will trumps fate, so if you're warned, you can actually do something about it.)

I could also see an alliance between a banshee and a valkyrie - the banshee figures out what important people are going to die when, and the valkyrie goes to collect...

At a minimum, there is no way a character lacking free will should be able to buy out of a compel on their high concept.

I disagree, actually.  Buying out of a compel doesn't always mean an in-character excercise of free will - it just means that, right now, that aspect isn't going to complicate your character's existence.

For example: Say you've got an aspect "A sucker for a pretty face" - and a mysterious lady has just asked you to help - she needs a ride home, say.  But you probably don't have time to do that and still be on time to a date with your girlfriend - and she's unlikely to be sympathetic to the excuse... This is fairly obviously a compel, and while you could buy it off and leave this random person standing by the side of the road waiting for someone else... that's not really in character now is it?  Instead, you'd buy off the compel - and what do you know, the lights line up just right and you manage to make it to your date just barely on time.

Is buying off that compel an exercise of free will?  Of course not.  It's a mechanic for narrative control.

So, for the Black Court Vampire PC, of course you can buy off the compel to kill whoever it is - your character has plans, and this person is still useful... until he isn't.  And then you eat him.  But that's for later.