Stacy is Nathaniel Smith's daughter, the other half of Sunders & Smith, the Nate that substitute the MC's father. It makes sense that the scorned mother (Melissa) didn't keep the POS name, while the daughter does.
Why would they get a fraction of the inheritance? No clue. None of those should inherit anything, besides the Uncle. At least not without a strong reason for it.
About the freaking house, it seems pretty clear to me, from all the "Life Lessons" and the open decision, that it was a test placed upon the MC on how he would deal with the conundrum. You'll notice that there no choice to NOT give away the house, only of what to do with the Cheque. Never to cash it, but how to dispose of it. So, it is an integral part of the story.
It may seem like a weak move, but, bear with me here, what if it was the condition to unblock the rest of the Fortune? That he showed mercy and care to the "One that got away"? There's a lot more to know about the MC's father plans for the MC. The conditions to fulfill the 2 years later part of the will.
So far, I feel that the story, while still over 90% under wraps, makes a lot of sense. Would a responsible father throw 6.2 B to the hands of a teen that had the plan to spend "at least" one year partying before college? With no goals? No ambitions? With the company of someone that said father considered a bad influence? Or, would that father, orchestrate the best he could, for him to know and involve himself with the Best person he knew, the one with which he wanted to build his dreams, in order to have that son, grow and develop?
You are all up in arms about a paltry house when there's a veritable treasure, and I don't mean only money, just under his reach. I think you are all missing the forest because of the trees.
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Peace