In the remake, The Devil shows up after Elliot lusts for someone else's woman. A violation of one of the Ten Commandments.
If I recall correctly, Elliot mentions at the beginning of the film that Alison's recently broken up with her boyfriend. She's back together with her boyfriend at the end because of Elliot's own wish that she have a happy life.
Arguable. Elliot points out at the start of the film that Alison's romance has been "over for months now", so it's not unreasonable to think she's moved on. Furthermore, it seems more likely the situation was Fridge Brilliance of a different kind; namely that the devil, as a being of pure evil, is incapable of doing good. Thus, if someone makes a truly virtuous, selfless act, the devil would be unable to grant it; so, instead, she wrote an escape clause in the contract, voiding the whole thing if anyone tried to make her do so. It's less likely that the devil actually gave Allison a happy life and more that she simply refused (or was incapable of granting) Elliot's wish.
Alternatively, Alison is gay. She never actually says that she's dating a man, after all, just "seeing someone". Plus, if we go with the Old Testament idea of homosexuality being a sin, then if the Devil gave her the courage to accept that part of herself, than it would fall within the Devils ability to tempt towards sin, since Alison would be committing sins more often. Actually, that might be Fridge Horror...
Well, nothing in the movie implies she is gay. At the end of the day, all we know of Alison is from Elliot's perspective which is honestly very limited. He truly has not idea of her relationship status beyond what he has overheard. In all of the wishes, Alison is a concoction of the devil.
It may also be worth noting that when Elliot learns that Alison is seeing someone, he refers to her unknown partner as 'he' and she doesn't correct him.
And her diary has he wanting a sensitive man.
If Elliot is such a Nice Guy (if an Extreme Doormat who Desperately Craves Affection), why did the Devil consider him such a good candidate to tempt into evil to steal his soul? As Elliot himself realizes near the end of the movie, he's a bit of a closet jerk who wants to use his wishes to make his own life better rather than help others (though even he has standards, like he won't be a drug dealer), and is willing to magically brainwash Allison into loving him (or rewrite her personality altogether) without any regard for her consent or free will, and he doesn't show any interest in getting to know her as a person (her past, hopes, dreams, interests, or hobbies) other than her interest in him. (Though he convinces himself that he "knows her" since he knows random details like "she likes her bagels buttered, not toasted.") Part of his Character Development is realizing how selfish he's been (even his "noble" wish to be President is still self-serving since he gets fame and glory by being president) and how little he actually knows Allison or considers her own feelings, and it's only at this point that the Devil is unable to tempt him or take his soul.
For that matter, if we go with the Alternate Character Interpretation that the Devil is trying to provide a Secret Test of Character or stealthily teach him a lesson, then in most of his wishes, Elliot's plan to magically force Allison to end up with him falls apart when Allison's own feelings of the situation don't line up with what he wanted. He wishes they were married? She hates his guts. He wishes he were "the most emotionally sensitive man in the world" and that she loves him? His constant weeping and neediness drives her crazy. He wishes he were a big basketball star? She's turned off by his small package (aka she's not physically attracted to him). He wishes to sweep her off her feet with charm, wit, and sophistication? She's attracted to him, but he's not attracted to her. He wishes she'd respect him as president? She does, but even as president he's not infallible since he's susceptible to assassination. In all cases, he can't fully control her, her emotions, or the external situation since people don't (and shouldn't) work like that. It helps him realize that he's Loving a Shadow and he can't/shouldn't try to magically force Allison to be with him. Which leads to I Want My Beloved to Be Happy.
Bordering on A Wizard Did It, but: When Elliot wishes to be a "sensitive guy" and starts playing an acoustic guitar for Alison, he places his hands above the capo while changing chords, which shouldn't work because the capo would normally silence those notes... yet the correct chord plays on the soundtrack anyway. There's never been any prior indication Elliot plays guitar, so one probably just appeared in the wish because he associates the instrument with sensitive artists - he's probably seen a capo somewhere, but has no idea what they actually do. Meanwhile, the Devil knows he can't play, and she doesn't want the wish to go wrong just yet, so she's probably done something to the guitar so that it magically plays a pleasing chord progression no matter what Elliot actually does with it.
In the remake, the escape clause has made me wonder. If it requires a selfless wish, then no one who has read and understood the contract could escape it. Think on it, a person who knows they need to be selfless will be looking for a way to help themselves in the long run. They are out to save themselves and that is not selfless. Thus anyone who tries to understand the full extent of the deal ends up losing the only escape clause.
It could probably work if the selfless act was spontaneous.
Like in Constantine; he couldn't get out of going to Hell by just saving people because everything he was doing was basically about trying to earn forgiveness, but when he asked for Isabel to go to Heaven even when he knew that meant he'd be taken to Hell, Constantine was allowed to ascend into Heaven before Satan healed him.
The Devil's nightclub serves Elliot a plate of the same cookies his grandmother used to bake. But how would they have gotten the recipe unless Grandma went to...?
In his review, Roger Ebert questions why the devil goes out of her way to be a Jackass Genie, causing Elliot to want out of the deal. Wouldn't it be better for her to genuinely make his life wonderful, so he will recommend her services to others? Arguably applies to the original film as well, but there the devil had a concrete goal and doesn't need to keep drumming up new business in perpetuity.
Evil Is Petty probably. Also the ending suggests she's not that bothered with losing the soul in the end.
She's trying to make him use up the wishes as quickly as possible, so that she can get his soul.
If Elliot gets what he wants perfectly the first time around, he's less likely to have a need for the other wishes. Each time he screws up, he will want to fix it immediately. This is the logic of the film Wishmaster wherein the evil genie tends to grant the first wish affably, which encourages the person to make a second wish, which the genie makes go horribly wrong, forcing them to use the third wish to fix the issue.
This being the devil and all, she simply may be incapable of granting truly virtuous wishes that improve people's lives. Hence why she wrote an escape clause into the contract that triggers when someone makes a wish so pure-hearted, no amount of Jackass Genie-ing on her part could screw it up.
There is also an additional option. The Devil is not evil incarnate. The devil isn't out to damn the world. If we listen to the end the Devil may be just trying to teach Elliot and by extension humanity. It's all just a big lesson and not one done out of spite or ill intent. The devil doesn't lose in the end.
It fits with the more classic, Old Testament version of the Devil, who messes with humanity only with God's permission in order to test them.
