With a fragile aspect, I wouldn't really bother overcoming it. It goes away pretty quickly. But that's me.
I don't know if I'd use the word "immobilized". Instead, I might use the an aspect like 'quagmire' to represent the concrete melting and the person getting stuck in it. Then you could invoke the aspect to compel the person to be stuck/immobilized.
Slowed is probably a better aspect. If they want to immobilize someone, I agree, the better option is to use a block ... but blocks don't last as long as maneuvers. There isn't a correct way to do it. Just preference. Using an aspect that gets compelled has the risk of failing if the target pays off the compel and a block can be overcome by a sprint roll.
Regarding the concrete:
It really depends what kind of earth magic he's doing. Is taking dirt from the surrounding area and burying the person? Is he smashing up the concrete? I might require a more powerful effect if he's pulling up the concrete. Maybe it requires a minimum of 4 shifts of power instead of 3. 3 is just the base minimum to have an effect from a spell, but some maneuvers might be harder than others to do. Really, it's up to you as a GM to decide. Really, really powerful maneuvers might have catastrophic side-effects. You can compel those aspects created by players to complicate their lives.
As far as overcoming an aspect goes, as long as something is maintaining the maneuver, the person gets to resist and the target has to overcome. If it's a sticky aspect created by a spell then it's probably safe to assume the spell is maintaing the effect and they'd have to overcome the original power. If it's a person maintaining it (like an arm lock), when the target tries to overcome the armlock, the attacker gets to resist to prevent the target from escaping.
A scene aspect that doesn't have anything maintaining it probably has a base difficulty.
Regarding the -2 shifts. RANGE is sight. Targeting every person in a ZONE requires additional 2 shifts. But I wouldn't allow zone-wide maneuvers. Instead, I'd recommend using the spray attack rules for maneuvers if you want to target multiple people.