The aasimar alternate racial trait scion of humanity can't grant access to human alternate racial traits

Up front: Don't confuse race traits with racial traits. Human (and aasimar) race traits are what the aasimar now qualifies for because of the alternate racial trait scion of humanity. On the other hand, racial traits are those specific facets of the creature's race that can be picked and traded for other facets of the creature's race because of the creature's race.

So, although the aasiamar has picked the alternate racial trait scion of humanity, the aasimar's race nonetheless remains aasiamar. Hence, an aasiamar that takes the alternate racial trait scion of humanity can no more pick human racial traits than a half-orc can pick human racial traits. An aasimar with the alternate racial trait scion of humanity could take as its two race traits Aspiring Bard and Bralani's Step, but the same aasimar couldn't take the human alternate racial traits heart of the sea and heroic.

So while the alternate racial trait scion of humanity says

Some aasimars' heavenly ancestry is extremely distant. An aasimar with this racial trait counts as an outsider (native) and a humanoid (human) for any effect related to race, including feat prerequisites and spells that affect humanoids. She can pass for human without using the Disguise skill. This racial trait replaces the Celestial language and alters the native subtype.

...this alternate racial trait doesn't give the aasimar anything the aasimar didn't already possess. Herolab shouldn't've allowed such an aasimar to pick the human alternate racial trait dual talent, first, because the aasimar's not a human, and, second, because the aasimar doesn't have the things the alternate racial trait dual talent needs to replace. That is, the alternate racial trait dual talent says that it

replaces the +2 bonus to any one ability score, the bonus feat, and the skilled traits.

And the aasimar, even if the GM allows the aasimar to take human alternate racial traits, doesn't have those other traits to trade away for this trait, making the aasimar ineligible for the new human trait, just as the aasimar would be ineligible for a feat that had as a prerequisite human and Str 70 (assuming the aasimar didn't have Str 70, obviously). Even were the aasimar to meet the human prerequisite—which it does—, it can't meet the remaining prerequisites.

Aasimar variants' ability score bonuses replace the typical aasimar ability score bonuses

That is, an aasimar gets either the typical ability score bonuses or the ability score bonuses for the variant he rolls or picks:

Players may choose one of the following six heritages for their aasimar characters in place of the traditional aasimar racial features. Each heritage presents new ability modifiers, spell-like abilities, and skill modifiers that replace the default aasimar racial traits, as well as a pair of custom traits. Each entry also discusses the most common (though by no means ubiquitous) personality traits, physical features, and places of origin of aasimars with that particular heritage.

Emphasis mine.

Balancing Your Feat

I don't think it's terribly well balanced; in most cases, it's actually weaker than a feat:

• If you take the first option twice, it's the same as a basic ASI.

• If you take either the second or third option, it is strictly weaker than Skilled (PHB p170), which allows choice of three skill or tool proficiencies.

• Take the first and last option, and it's weaker than Linguist (PHB p167), which provides +1 Intelligence, three languages, and the ability to create coded messages.

Feats are supposed to be powerful, and your design is watered down in comparison. Feats also tend to be packaged abilities with a theme, selecting one makes a character different from others with the same class. Were I playing in your game, I wouldn't take anything but the ASI, which I could do anyway.

There are also stacking concerns - normally, the same feat cannot be taken more than once. If this is the only feat available in your game, then it's clearly stackable. It allows a player (over time) to stack a large number of skill proficiencies, potentially having proficiency in everything, which will trivialize skill checks and reduce each character's uniqueness.

The Power of (Variant) Humanity

Variant human is pretty powerful, but so is a normal human. I think you're overlooking the raw strength of solid attributes. Having a bonus to six stats shores up potential weaknesses and/or saves points the player can spend on the character's desired strengths.

New Player Advice

If you're trying to ease new players into the system, you could disallow "Variant Human" completely. That would push off selection of feats until L4 at the earliest (if you allow them at all, they're entirely optional), and allow some time for people to get used to the basics.