Harry's statement regarding Earth Magic isn't actually a claim that HE finds it harder, but that the magic is harder in general. I'd love the exact quote, to see if he really is talking about it through his experience, or if he mentions it being less used by other wizards due to the difficulty.
If it's his own view - "It's harder because you have to do X instead of Y" can still be his own view, as he isn't thinking about his own trained powers in Y" - that's pretty easy to cover with the refinement differences. If he's talking about others, well, I still don't think that's a good enough reason to make it more of a bitch for anyone to take Earth over anything else... there isn't a real mechanical benefit to earth, so why penalize it?
As for the player who wanted to be able to shut off people from him: if he raised a wall it should be treated just like increasing a border, which I am pretty sure there's a spell strength for (isn't it just a block, really?). Logically, yes, if he threw up a wall that's block 4 and you stated that this means it's a solid wall 10 feet high, then he has cut off line of sight and the bad guy shouldn't be able to fling a spell at him or fire a gun until he is back in sight. So the bad guy jumps on a garbage can and now he can see you. Or he blasts through the wall with magic (overcoming the block strength with a spell) or gets you by targeting the entire zone that the wall is connected to on your side (hey, he can see some part of that zone; above your head is legit.) The werewolf just scales it.
Plus, that wall you throw up is evocation, so it literally justs lasts one action unless you pump some duration into it. I mean, if he rips up the cobblestones and stacks them in a wall, they may not topple instantly but they will be super easy to kick down once the magic fades (no mortar). If he summons raw earth to put there "I make a big stone", then his big stone crumbles into dust pretty easy if hit. There's no reason earth magic SHOULD get a permanent benefit (like permanent walls) that another element wouldn't get. You could be generous and leave an aspect up over the area, that can be tagged as needed and give a potential boost to the block strength there.
Hrm.. compare that to fire... you could make a block 10 wall of fire. A round later, the wall just poof, dies out, but now the ground and walls around it may have caught fire. You can give it the famous "on fire" aspect and can that be tagged to make passing it harder.
Really, you're being generous, because his block action isn't really a declaration so he's not really creating an aspect, and he could be charged an extra 4 shifts to have a sticky aspect hang around. But I kinda see treating the damage/long term effect as an aspect as working pretty well.
Anyways, he can block of pursuit or block line of vision. It's just very short duration, and all he's doing is forcing the opponent to take some other action to get to him. Seems legit. The only issue I see is just making sure he knows that he isn't creating a permanent object, because that's not what evocation - even earth evocation - does.