You say that this story was returned with the editor complaining about punctuation and "mechanics." Punctuation I'm not interested in. I'm no authority and turn all my manuscripts over to a professional proof-reader before submitting them. "Mechanics," is a fairly ambiguous term but I'm going to interpret it as referring to your narrative technique. You have a good idea here and some really nice visuals, but you're not telling the story in a way that produces the kind of effect you seek.

First off, look at your opening sentence.

Alan’s Gucci leather shoes tapped out a staccato on the concrete sidewalk as he strode towards the squat guard house straddling the roadway into “Flowering Springs Retirement Community”.

This is what I call a "big tent" sentence: Come on in; there's room for everything. You offer us Alan's choice in shoes (Gucci) that make a staccato tapping on the concrete sidewalk. Not bad, image-wise, though staccato suggests someone moving rapidly, in a hurry, and Alan's subsequent interaction with Dan doesn't play that way at all. But note what else is in the same sentence: a squat guard house, the fact that it straddles the roadway, and also the fact that it leads to "Fowering Springs Retirement Community." None of it is wrong, just unwieldy. At least two sentences would get the job done with more focus. For instance,

Alan’s Gucci leather shoes tapped out a staccato on the concrete sidewalk. Ahead, a squat guard house straddled the entrance to “Flowering Springs Retirement Community”.

Aside from separating the two thoughts into their own sentences, you trade straddling, which may seem like a verb but is really a modifier for guard house for the far stronger verb, straddled. Same content, but it just sounds better.

Let's look at the rest of your opening paragraph:

Pausing beside the slim, wooden gate draped across the road, he tapped out a cigarette and lit up. Alan studied the tall, granite walls surrounding the small compound and watched the brilliant wedge of morning sun top the run down apartments across the street and toss its sparkling rays on the razor wire crowning the communities gothic barrier.

It's hard to know where to start with this paragraph. The most important flaw is the unnecessary effort you make telling us that Alan studies "the tall granite walls surrounding the compound..." You also tell us that he watches the sun rise." Never do that again. You're writing in a third person restricted POV. By definition, anything that is seen, heard, felt smelt or thought is done so through the filter of Alan's perceptions. If you tell us that a tall granite wall surrounds the community, we know (because we're clever and smart) that Alan is the one noting this fact. Get rid of the "he studied," and "he watched" and all those other narrative buffers that stand between the reader and the things that they can figure out on their own.

But here's the bigger problem: it all needs to go. Also the bulk of the interaction with Dan. You eat up 361 words and haven't even begun the story yet. And when you do, you cheat. You tell us about it. Show, don't tell is quite often horribly misplaced advice. But not here. Telling us that Alan is a con man has zero impact on your reader. Far more effective to put Alan in the midst of the situation, give us the same sense data that you offer now in your opening, but with a purpose, namely, setting up the actual story. The present opening is just marking time.

Once you get down to cases, things move a bit more swiftly, and with a sense of focus. However, I strongly recommend that you spend those 360+ words, and maybe more, giving us far more access to the relationship between Alan and Grace. In its present form, you have a sharp line of demarcation between This is a con routine and then, with no transition at all, This is a nightmare. You need to bring your reader along with you on this arc. It is the movement from one state to another that will provide the interest. You need to salt a few clues along the way, much as you've done with the reference to Cezanne, although not as blunt. Once we hear Grace say that, we know that all bets are off. Instead, you want your reader to puzzle over the situation, much as you want Alan to puzzle over his predicament. There needs to be a progression from Well, that's strange, to Hmmm, that's really strange, only arriving at Oh my GOD, WHAT'S HAPPENING when the ground has been properly prepared.

You've given yourself one useful device to exploit. I call it the decoy story. Readers need a story to stay interested. It doesn't have to be the real story. It can be something that feels like it's the story, but which is discarded once you're ready for the big reveal. In your case, you might consider giving Grace some healthy suspicions about the quality and true value of the work for which she is supposed to be writing a check. Have them spar a bit, so that you never need to be so obvious as to take the reader by the hand and tell them that Alan's a con man. They'll put things together on their own, and they'll be perfectly content thinking that they're reading the real story, as long as you keep ratcheting up the tension and conflict.

Then, when the real story kicks in, you'll take the reader by surprise, much as Alan will be surprised. And then, they'll react, not with an "Oh, I get it," response which is what you get now, but they'll be white-knuckling their way through to the end.

And then your editor will publish it.