Obviously the "MC is John's son" theory has been going on for a while, and while I believe it possible, I just didn't see how until your comment. I think most likely, John impregnated MC's real mother as one of his business transactions/employees, but he met and married Nicole around the same time. After "Momma" gives birth, she's not able to keep the child (death?) and of course John can't disclose his infidelity to Nicole (who is pregnant with Chloe, whom I believe is his real daughter) so he can't bring MC home, and utilizes one of his business deals with Hal and Jenny to raise MC until adulthood (thus why they feel urgency in revealing their association with John). After the near disaster with Momma, John starts implementing the temporary sterilization shots. Though after Chloe is born, John loses his lust for Nicole, thus closing off any option for a legitimate son, or he decided from the beginning that since MC is his eldest son, he will inherit Magic Grand, thus John abstains from Nicole to ensure that she never gives birth to a son who could then challenge John's chosen heir.
Fast-forward to shortly before the game, either John didn't realize that Chloe has started dating his son, or I think John actually encouraged it, as either way: John uses this as the platform to get MC "hired" at Magic Grand. This explains why despite MC potentially breaking up with Chloe, MC's investigations, challenges, and interrogations, thereby becoming a threat at worst, or a nuisance at best, plus the affair, John doesn't fire MC, instead he tests the young man's responses, and ensures that MC "stays in his lane" by not quiting, but not getting too much information. This also explains why at the dinner with Sylvia, 1) John has MC watch Sylvia change (counting on him seeing her naked), 2) John states/confirms that MC's relationship with Chloe will end soon, thus 3) MC should look elsewhere, like John's own woman: Sylvia (either the first of John's inheritance, or testing how much similarity MC shares with his father regarding taste in women/willingness to take a vulnerable woman like Sylvia).