The difference between a Compel and a high-strength block can only ever be crystal-clear.  Compels are negotiated, and as such, cannot be proposed, accepted, or otherwise implemented without being made clear to all parties that such is happening.

Yep, as good as the DFrpg is, there are still some stupid statements in YS.
That philosophy runs directly counter to the very core foundations of the system, and is incompatible with its primary mechanic (the FP economy).

Or, to put it another way: If Spiderman exists in the DFrpg (which, according to Bob, for all intents and purposes he sort of does), then so does Nightcrawler (for all intents and purposes, sort of).

If blowing things up might plausibly be (or seem to be) the 'right choice' for that situation, then they deserve the Compel.
If they're simply frustrated while trying to open a jar of jam, and want to blow THAT up, then yeah, the aspect is probably inappropriate.

Remember, though, that those 'Blind Spots' should typically be represented with Compels, even (perhaps especially) if they're self-imposed and self-policed.

If you're using the standard element system, well, it really depends what you're trying to 'heal'.
Fire purifies, but won't be nearly as much help when dealing with that broken wrist.
Earth abides (again, useful when dealing with infections and the like), but once broken tends to remain so (again, less helpful with injury).
Air quickens.  Whatever the sickness or injury, it might be made to run its course faster, whatever that course might be (best hope the patient was going to pull through on their own in the first place if this is your sole recourse).
Spirit is the stuff of Will made manifest; it'll help with just about anything, but likely is not the best at any of it.

If the power is provided by the Pantheon as a whole, I might go with something along the lines of:

The Greek Pantheon is powerful and diverse; those Sponsored by them may gain access to nearly any form of ritual magic for the price of one point of Debt for each ritual that would not otherwise be available.
The Greek Pantheon abides by its own laws; those Sponsored by them may take a point of Debt to avoid any deleterious effects of Lawbreaking for one spell.
The will of the Greek Pantheon is powerful indeed, even as expressed through their proxies; downgrade the Toughness abilities of any supernatural creature by one step when applying them to spells cast using this Sponsored Magic.

(the first two are basically just additional ways to spend Debt, and so are worth very little: you pay for them as you use them, or at least shortly thereafter)

I sometimes don't give a target number if time is a factor.  I'll say it's a Lore extended test.  Each roll is one day and then let them spend the time...it adds tension if they don't know they'll succeed on time

It adds tension to the game, though, by robbing it (or other factors) from the story.