Because it will cause limits at the levels that most people will play the game.

Isnt that a normal rule?
Or do you mean by "in the head" like [...]

The rules imply it, I just went ahead and made it explicit.  Harry does something similar in, erm, White Night I think, when using a spell to contact Elaine.  It's also why I throw in the comment about not being able to drive while doing that...

I like the 5 minutes of preparations HR.So you would need an Workspace 3 to use +2 skills.

The stuff about workspace is mostly there to make the High Quality Workspace stunt explicitly worth something.

Question: Do failed declarations carry an penalty besides lost time?

No. 

That I dont like. Anther needed skill for Wizards? (Discipline, Lore, Conviction, Resources, Endurance)

That rule is there for a few different reasons.
1
To help keep a cap on guys who want to do huge thaumaturgy when they have several days to do nothing.

2
To make the various shortcuts to power slightly more attractive.  If there's no real cap on how long you can do the ritual, you can just sit down and do a power 40 ritual 'safely'.  And note that being restricted is only worth a -1.  So guys like Harry with a higher endurance than discipline can do rituals for as long as they want to, really.

3
To prevent the PC from just doing the, "well I have discipline 5" so can just draw one power an exchange for the whole thing...

Not shure I like the 5 Minutes per exchange rule. (But I like it better than speed of plot or how can I fasttalk the GM into as little time as possible.)

When first writing, I hadn't intended it to be a literal five minutes per pass.  For example, if the PC pulls of some shenanigans to do the whole ritual in one pass, then you can just declare that it took "exactly 2 minutes and 47 seconds" or something.  But when you want to know about how long the whole multipass ritual took, saying that each power draw takes "a few minutes" would only confuse matters.

I got the impression that the arrangement at Mac's was mostly to get rid of 'accidental' power summoning.  Remember that there have been plenty of times that Molly has gotten angry of something, and Harry has mentioned dispersing the power she'd called up.  So I'd assume that a lot of the non-wizards who drink at Mac's also do the same thing. 

The way I'm currently doing evocation based maneuvers is that extra shifts allocated to duration define a number of exchanges where the target is not allowed to attempt a (counter) maneuver to remove the aspect that the (first) maneuver placed on them.

I chose to do it this way because non-spell based maneuvers don't have to have any shifts assigned to duration (expect for the first shift to make them sticky) still last for the whole scene. 

So if I use a mundane maneuver to give you sand in the eyes you could always take an action yourself to clear the aspect.  If I use a maneuver to place magical glop over your eyes and spend 3 shifts for duration, you can't even attempt to remove the aspect until the duration is up, and then you can spend an action to remove the aspect normally, just like you could vs the sand.

First Spell

------------
[...]
Alan decides to put the aspect "Monstrously Strong" on Bobby for a scene.
[...]

This sounds like a maneuver, which would be difficulty 3 against an unresisting target, I think.  It would normally last "a few minutes" (one scene), and any more time you wanted it to last, you'd need to add more shifts for power (see the time chart, YS 315)  So making it last the whole day would require another 6 shifts of power.
You could reduce the complexity by making the potion inflict a consequence when it wore off.  For example, if you wanted it to last all day (normally a power 9 potion, but made is cause a moderate physical consequence when it wore off, then it would only require power 5 to create, since 4 shifts of power would come from the consequence.

And yes, they would only get 1 free tag on the aspect.

Does the complexity of the spell increase for tougher individuals (the
way it does for spells which are meant to "take out" a la the Heart Exploding Hex from Storm Front)?

Not if the target doesn't resist.

What are alternative approaches to accomplishing the same goal? Either
math wise, theme wise, or mechanics wise?

We could declare the potion as an effective might maneuver for how much you can lift.  We'll call it power 6.  So instead of using your might, you could just take a 6 whenever it came to lifting stuff.  But that would only last 1 exchange "a few moments' as it's base time.  If we wanted to make it last around an hour, that would be 6 steps up the time chart.  If we go with moderate consequences again, that would make the final potion power 8.

Imho, magical effects should only be able to do one 'part' of a skill at the time.  So, for example, you could make the Might potion that let you lift things, or a Might potion that made you a great wrestler, but not a potion that just made your might skill higher for a while.

Evocation blocks are rolled...but it's purely a control roll, anything in excess of the Power channeled is wasted.

My question was along the lines of this:
Do I roll the dice to modify the power of the block, like if I were defending with athletics?
In the example combat, when Jing-Wei is dodging gunfire fire with lore; she rolls the dice each time there is an attack and modifies her lore each time.
That seems to be the right way to do things, so my question was, do I do the same roll off for evocation defenses?  Or do you not roll the dice for them?

As for the rest, looks like we disagree about rules complexity.

But more generally, are there any places in my house rules that are unclear?  Do there seem to be any way to exploit them?  Is there anything in them that seems to
contradict the fluff?

For example, can the Merlin still stop the Red court with a ward, while combat goes on?
Using your writeup, the Merlin has a base complexity for wards of 18, and a base control of 11 (and an base ritual power of 7).  If we assume that a complexity 18 ward is enough, he can move on to the ritual right away.  With 5 minutes of prep, he could in theory make 8 declarations, which should give him plenty more power if he wants it.

But what about getting the ritual done in a speedy manner?  Assuming none of the senior council are helping him with the ritual, he can safely throw up a complexity 18 ward by himself in about 15 minutes.  But the book says that the Gatekeeper was there to help.  Assuming the Gatekeeper can safely call up around 5 power an exchange for wards, that would cut their safe time down to around 10 minutes (2 power draws).  But what if they wanted to be unsafe, and call up power closer to their discipline?  Then they can do the whole thing in around 5 minutes.

What if a complexity 18 ward isn't going to be enough?  (And they were as desperate as PCs tend to be?)  They could take 5 minutes of prep and get another 12+ complexity, especially with the Gatekeeper to contribute (they can each trivially generate 3 successful declarations).  But how long will it take them to throw up?  We'll say they each have 3 fate points to spend here, that means the Merlin could call up around 15 power and the gatekeeper maybe 10 in the first ritual pass.  Then they can safely complete the ward in the next pass.  If they are truly PC level desperate (and have plenty of fate points), they might be able to do the whole thing in one pass, after taking a plenty of consequences for drawing that much power in one pass.
So, if they are really desperate, the Merlin and the Gatekeeper can do a force 30 or so ward in around 10 minutes.

That's close enough to the time a large combat might be going on in the background for me.

What about the thing Harry does with the ghosts in Grave Peril?  The book lists it as complexity 5.  Harry has a good lore at this point, so he can either accept a minor consequence or invoke an aspect to be able to move directly into the ritual, and he can do it in one pass.  So he can have it done in 'around 5 minutes'.  Since the vampires had fallen back from the battle for a moment, that's good enough for me.

Could the times that Harry says "there wasn't time to get the shield up" be justified in game as people ambushing him?  There are plenty of times that Harry is able to call up the shield to deflect something that would normally have initiative on him.