I think conscious intent is a big part of the invitation process, so I'd lean towards 'no' unless presented with a compelling argument to the contrary.

What makes you think he'd have to call on Mab for more juice?  Why wouldn't he just call on Winter?

Or, in other words, I would absolutely allow, with sufficient justification, a character to purchase capability with Sponsored Magic without having an embodied, personified, or even known sponsor.
edit: for clarification, I would not place any mechanical restrictions on this power, but the Compels would likely be different from those against a character dealing directly with a known personified entity acting as a sponsor

Rain of Blows (Fists): Whenever you could make a Fists attack against an opponent, you may instead make two such attacks, each at a -1 penalty.

You're going to need a heck of a lot more than a stunt to justify shattering the action economy to bits.

And I'd venture to say that trying to change the past isn't going to be the same sort of spell as just looking at the past, any more than watching a TV broadcast of a parade in progress is going to let you derail that parade.

It might derail the parade if a TV camera materialized out of thin air in plain view, but there's no real need for that, and most practitioners probably aren't that sloppy.

Except there are plenty of ways to make those inferences already without using magic, so why would using magic to do it and cut out the middle man be disallowed?  Its not really that different than watching security footage of the event in question.

Not that I disagree with your conclusion, but your argument is terrible.  There are plenty of mundane ways to kill people, too.  That doesn't put a stop to the 1st Law.

Or more accurately, a character who knows if he picks interesting, easily compellable aspects, he won't be penalised for wanting to make an interesting character. Something which I do not think the core rules support, because I do not believe compels are actually cost-neutral.

A character who refuses all compels without repurcussion is indistinguishable, narratively, from a character who is never compelled.
We have established that characters that are never compelled are 'boring', and undesirable.
Thus, characters refusing compels must be disinsentivized.

The cost-neutrality of compels matters only for accepted compels.  The game strives, and must strive, not to avoid penalizing characters for being presented with compels, but rather for accepting them.

Are those 'affect area' attacks considered to automatically hit, & targets just defend against damage?

It depends entirely on the narrative.
There is no mechanically-backed criteria for determining whether or not an attack 'hits'.
A 'successful' attack that inflicts stress may only have come close enough to 'hitting' its target that it required some meaningful exertion to avoid.
A 'successful' attack that inflicts consequences may have done so as a result of the subjects efforts to avoid the attack itself (eg. grinding rocks and dirt into your palms and knees as you dive for cover to avoid the rocket aimed at your head).

Yes, refused compels are expensive.  You're the only one that thinks this is controversial.