Sorry, I thought the person who was assumed dead was the father, not the character receiving the shares. If the person receiving the shares was the one assumed dead, it makes way more sense as to why it was less believable. It's still possible but, unless things were set-up in a way that already assumed that everyone would be dead, except for that one guy, who is still totally alive but is going to be missing for a while, then it's highly unlikely.
Then again, this is a universe where magic exists, so a wizard did it.

Depending on how long MC was presumed dead, and if there were any issues with either his estate, or of his father, the shares could be held by the personal representative of either estate for quite some time. If there was any less than 7 years between when his father died, and when he came back, then it's completely reasonable that he would be entitled to those shares. The longer it is after that seven years, the less likely it is that he would have the shares. The shares could've also been held by another entity, like a family trust or holding company. And as long as a trustee, or a controlling interest in the holding company, was still alive and willing, they could give him back the shares.

But: He wouldn't have an officer or director position in the main company if he was presumed dead. Directors need to be re-appointed every year, and depending on the set-up of the corporation, you sometimes need to be a director in order to be an officer, or the officers need to be reappointed every year. A position like "chairman" was likely appointed to someone else rather quickly after the presumed death. Then again, it might also have the wildest bylaws ever written, likely by a wizard.